Did most of India’s ancient texts really survive and where can one find and read the original texts?

I’ve always been curious about how much of India’s oldest literature still exists today. We still have the Vedas, Upanishads, Ramayana, Mahabharata and other texts, even though so many ancient libraries and manuscripts were lost over time.

We often hear that many old manuscripts were lost in events like the Nalanda University burning, but these major works still seem to be around in full.

How much of this early literature has actually survived in its authentic form? Were big portions lost, or mainly the commentaries and regional branches?