How Buddha practiced meditation himself

On arrival, they saw the Blessed One sitting in the grove — his legs folded crosswise, his body set straight, mindfulness established to the fore. On seeing him, they went to the brahman of the Bharadvaja clan and, on arrival, said to him, “Sir, you should know that Gotama the contemplative is in that grove over there, sitting with his legs folded crosswise, his body set straight, mindfulness established to the fore.”
So the brahman of the Bharadvaja clan, together with the youths, went to the forest grove. On arrival, he saw the Blessed One sitting in the grove — his legs folded crosswise, his body set straight, mindfulness established to the fore. On seeing him, he went to the Blessed One and, on arrival, addressed him in verse:

In the deep-looking forest,
teeming with terrors,
having plunged into the wilderness
— desolate, empty —
unflinchingly, steadfastly, compellingly,
you practice jhana, monk:
How very lovely you look!

SN 7.18

On Abhidharma

Numerous Abhidharma traditions arose in India, roughly during the period from the 2nd or 3rd Century BCE to the 5th Century CE. The 7th-century Chinese pilgrim Xuanzang reportedly collected Abhidharma texts from seven different traditions. The various Abhidhammic traditions have very fundamental philosophical disagreements with each other. These various Abhidhammic theories were (together with differences in Vinaya) the major cause for the majority of splits in the monastic Sangha, which resulted in the fragmented early Buddhist landscape of the 18 Early Buddhist Schools.

In the modern era, only the Abhidharmas of the Sarvāstivādins and the Theravādins have survived intact, each consisting of seven books, with the addition of the Sariputra Abhidharma. The Theravāda Abhidharma, the Abhidhamma Pitaka (discussed below), is preserved in Pāli, while the Sarvāstivādin Abhidharma is mostly preserved only in Chinese – the (likely Sanskrit) original texts having been lost, though some Tibetan texts are still extant. A small number of other Abhidharma texts of unknown origin are preserved in translation in the Chinese canon. These different traditions have some similarities, suggesting either interaction between groups or some common ground antedating the separation of the schools.

Also, the Pali version of the Abhidhamma is a strictly Theravada collection, and has little in common with the Abhidhamma works recognized by other Buddhist schools. The earliest texts of the Pali Canon have no mention of (the texts of) the Abhidhamma Pitaka. The various Abhidhamma philosophies of the various early schools have no agreement on doctrine.

For the Abhidharmikas, the ultimate components of existence, the elementary constituents of experience were called dharmas The “base upon which the entire [Abhidharma] system rests” is the ‘dharma theory’ and this theory ‘penetrated all the early schools’
These dharmas were seen as the ultimate entities or momentary events which make up the fabric of people’s experience of reality.

More on Dhamma theory:

Stopping of the six contact-media

Then Ven. Maha Kotthita went to Ven. Sariputta and, on arrival, exchanged courteous greetings with him. After an exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there, he said to Ven. Sariputta, “With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “With the remainderless stopping & fading of the six contact-media, is it the case that there is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “…is it the case that there both is & is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “…is it the case that there neither is nor is not anything else?”
[Sariputta:] “Don’t say that, my friend.”
[Maha Kotthita:] “Being asked if, with the remainderless stopping & fading of the six contact-media, there is anything else, you say, ‘Don’t say that, my friend.’ Being asked if … there is not anything else … there both is & is not anything else … there neither is nor is not anything else, you say, ‘Don’t say that, my friend.’ Now, how is the meaning of your words to be understood?”
[Sariputta:] “The statement, ‘With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?’ objectifies non-objectification. The statement, ‘… is it the case that there is not anything else … is it the case that there both is & is not anything else … is it the case that there neither is nor is not anything else?’ objectifies non-objectification. However far the six contact-media go, that is how far objectification goes. However far objectification goes, that is how far the six contact media go. With the remainderless fading & stopping of the six contact-media, there comes to be the stopping, the allaying of objectification.
“And suppose someone were to ask you, ‘This fire that has gone out in front of you, in which direction from here has it gone? East? West? North? Or south?’ Thus asked, how would you reply?”
“That doesn’t apply, Master Gotama. Any fire burning dependent on a sustenance of grass and timber, being unnourished — from having consumed that sustenance and not being offered any other — is classified simply as ‘out’ (unbound).”
“Even so, Vaccha, any physical form by which one describing the Tathagata would describe him: That the Tathagata has abandoned, its root destroyed, made like a palmyra stump, deprived of the conditions of development, not destined for future arising. Freed from the classification of form, Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea. ‘Reappears’ doesn’t apply. ‘Does not reappear’ doesn’t apply. ‘Both does & does not reappear’ doesn’t apply. ‘Neither reappears nor does not reappear’ doesn’t apply.

Tathagata cannot be described

When we feel the urge to reflect on qualities, motives of the Enlightened Being, when we try to envisage what his thought and emotions look like, we always should keep in mind that such a being is transcendent, indescribable and cannot be grasped by us.

At one time Venerable Sabhiya Kaccāna was staying at Nādika in the brick house. Then the wanderer Vacchagotta went up to him, and exchanged greetings with him. When the greetings and polite conversation were over, he sat down to one side, and said to Sabhiya Kaccāna:
“Master Kaccāna, does a Realized One exist after death?”
“Vaccha, this has not been declared by the Buddha.”
“Well then, does a Realized One not exist after death?”
“This too has not been declared by the Buddha.”
“Well then, does a Realized One both exist and not exist after death?”
“This has not been declared by the Buddha.”
“Well then, does a Realized One neither exist nor not exist after death?”
“This too has not been declared by the Buddha.”
“Master Kaccāna, when asked these questions, you say that this has not been declared by the Buddha. What’s the cause, what’s the reason why this has not been declared by the Buddha?”
“In order to describe him as ‘possessing form’ or ‘formless’ or ‘percipient’ or ‘non-percipient’ or ‘neither percipient nor non-percipient’, there must be some cause or reason for doing so. But if that cause and reason were to totally and utterly cease without anything left over, how could you describe him in any such terms?”
“Master Kaccāna, how long has it been since you went forth?”
“Not long, reverend: three years.”
“Well, you’ve learned a lot already, let alone what lies ahead!”

SN 44.11

About existing Buddha/Arahant after death

The Tathagata exists after death’ — this craving-standpoint, this perception-standpoint, this product of conceiving, this product of elaboration, this clinging-standpoint: That’s anguish.
‘The Tathagata doesn’t exist after death’
‘The Tathagata both does and doesn’t exist after death’
‘The Tathagata neither does nor doesn’t exist after death’ — this craving-standpoint, this perception-standpoint, this product of conceiving, this product of elaboration, this clinging-standpoint: That’s anguish.

https://www.accesstoinsight.org/tipitaka/an/an07/an07.051.than.html

“If there is the view ‘the world is finite,’ …
‘the world is infinite,’ …
‘the soul is the same as the body,’ …
‘the soul is one thing and the body another,’ …
‘after death a Tathāgata exists,’ …
‘after death a Tathāgata does not exist,’ the holy life cannot be lived …  
If there is the view ‘after death a Tathāgata both exists and does not exist,’ the holy life cannot be lived; and
if there is the view ‘after death a Tathāgata neither exists nor does not exist,’ the holy life cannot be lived.

Whether there is the view ‘after death a Tathāgata both exists and does not exist’ or the view ‘after death a Tathāgata neither exists nor does not exist,’ there is birth, there is ageing, there is death, there are sorrow, lamentation, pain, grief, and despair, the destruction of which I prescribe here and now.

“Therefore, Mālunkyāputta, remember what I have left undeclared as undeclared, and remember what I have declared as declared. And what have I left undeclared?
The world is eternal’—I have left undeclared.
‘The world is not eternal’—I have left undeclared.
‘The world is finite’—I have left undeclared.
‘The world is infinite’—I have left undeclared.
‘The soul is the same as the body’—I have left undeclared.
‘The soul is one thing and the body another’—I have left undeclared.
After death a Tathāgata exists’—I have left undeclared.
After death a Tathāgata does not exist’—I have left undeclared.
‘After death a Tathāgata both exists and does not exist’—I have left undeclared.
‘After death a Tathāgata neither exists nor does not exist’—I have left undeclared.

Why have I left that undeclared? Because it is unbeneficial, it does not belong to the fundamentals of the holy life, it does not lead to disenchantment, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. That is why I have left it undeclared

https://www.accesstoinsight.org/tipitaka/mn/mn.063.than.html

About Nirvana

There is, monks, an unborn — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that emancipation from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, emancipation from the born — become — made — fabricated is discerned.

Unanswered questions

“Mālunkyāputta, if there is the view ‘the world is eternal,’ the holy life cannot be lived; and if there is the view ‘the world is not eternal,’ the holy life cannot be lived. Whether there is the view ‘the world is eternal’ or the view ‘the world is not eternal,’ there is birth, there is ageing, there is death, there are sorrow, lamentation, pain, grief, and despair, the destruction of which I prescribe here and now.

“If there is the view ‘the world is finite,’…‘the world is infinite, ’…‘the soul is the same as the body,’…‘the soul is one thing and the body another,’…‘after death a Tathāgata exists,’…‘after death a Tathāgata does not exist,’ the holy life cannot be lived… If there is the view ‘after death a Tathāgata both exists and does not exist,’ the holy life cannot be lived; and if there is the view ‘after death a Tathāgata neither exists nor does not exist,’ the holy life cannot be lived. Whether there is the view ‘after death a Tathāgata both exists and does not exist’ or the view ‘after death a Tathāgata neither exists nor does not exist,’ there is birth, there is ageing, there is death, there are sorrow, lamentation, pain, grief, and despair, the destruction of which I prescribe here and now.

“Therefore, Mālunkyāputta, remember what I have left undeclared as undeclared, and remember what I have declared as declared. And what have I left undeclared? ‘The world is eternal’—I have left undeclared. ‘The world is not eternal’—I have left undeclared. ‘The world is finite’—I have left undeclared. ‘The world is infinite’—I have left undeclared. ‘The soul is the same as the body’—I have left undeclared. ‘The soul is one thing and the body another’—I have left undeclared. ‘After death a Tathāgata exists’—I have left undeclared. ‘After death a Tathāgata does not exist’—I have left undeclared. ‘After death a Tathāgata both exists and does not exist’—I have left undeclared. ‘After death a Tathāgata neither exists nor does not exist’—I have left undeclared.

“Why have I left that undeclared? Because it is unbeneficial, it does not belong to the fundamentals of the holy life, it does not lead to disenchantment, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. That is why I have left it undeclared.

MN 63