What Did Gautama Buddha Actually Do By Sitting Under The Bodhi Tree (Vipaśyanā)?

I’ll give you a very short answer. He did the practice of śamatha and vipaśyanā. He rediscovered vipaśyanā as he clearly said in the Saṁyutta Nikāya, Dhammacakkapavattana Sutta, and the Nagara Sutta. Various styles of śamatha were already in vogue in Greater South Asia long before he was born. Then he combined śamatha and vipaśyanā and reached the final Samyaksaṃbodhi in Bodhgayā, seeing through Interdependent Origination/Emptiness (Skt: pratītyasamutpāda/śūnyatā) and the Four Noble Truths (Skt: ārya satya).

There is a stock sentence in Saṁyutta Nikāya, Dhammachakkapavattana Vagga:

“vision arose, discernment arose, knowledge arose, illumination arose within me with regards to things never heard before”

taṁ kho panidaṁ dukkhanirodhagāminī paṭipadā ariyasaccaṁ bhāvitan’ti me, bhikkhave, pubbe ananussutesu dhammesu cakkhuṁ udapādi, ñāṇaṁ udapādi, paññā udapādi, vijjā udapādi, āloko udapādi.

Saṁyutta Nikāya, Dhammachakkapavattana Vagga 56.11

This has been repeated several times within this sutta.

In the Nagara sutta, Nidānasaṁyutta, Mahāvagga, Saṁyutta Nikāya, Bhagavān says:

“In the same way, I saw an ancient path, an ancient road, traveled by rightly self-awakened ones (Meaning Buddha’s) of the past. And what is that ancient path, the ancient road, traveled by rightly self-awakened ones? Just this noble eightfold path….”

(Evameva khvāhaṁ, bhikkhave, addasaṁ purāṇaṁ maggaṁ purāṇañjasaṁ pubbakehi sammāsambuddhehi anuyātaṁ. Katamo ca so, bhikkhave, purāṇamaggo purāṇañjaso pubbakehi sammāsambuddhehi anuyāto? Ayameva ariyo aṭṭhaṅgiko maggo…)

Nagara sutta, Nidānasaṁyutta, Mahāvagga, Saṁyutta Nikāya

Also, elsewhere he has clearly said that the way to become an Arhat is through various combinations of śamatha and vipaśyanā. For example, in Añguttara Nikāya, 4th Nipāta, Praṭipadā Vagga, Yuganaddha sutta lays out four ways to Arhathood, which are:

1. First śamatha then vipaśyanā is practiced a lot (samathapubbaṅgamaṁ vipassanaṁ bhāveti)
2. First vipaśyanā (vidarśanā) then śamatha is practiced a lot (vipassanāpubbaṅgamaṁ samathaṁ bhāveti)
3. Practices a lot the combination of both śamatha and vipaśyanā (samathavipassanaṁ yuganaddhaṁ bhāveti)
4. Remains in the state of mind (mānasa) arisen from the combination of śamatha and vipaśyanā, (dhammud­dha­­c­ca­vi­g­gahitaṁ mānasaṁ hoti); which is found mostly in the Mahāyāna systems.

The practice of all or any of the four ways can be used to attain the bodhi.