If you ask a Librarian to show you the section with the Upanishads and Sūtras, the nice lady is likely to point you to the shelves marked ‘Religion’.
But these compilations are documents of Inquiry characterized by Trial and Error, not claim and faith. It is in this very evolution, the error of experiment and application that Shūnyam takes form. They were the obsessions of the Learned. Your normal healthy farm-boy will find it all quite batty.
The search for some Grounding Truth is Man’s first investigative journey. And the First Inquirers were the men and women of Religion [from the Latin-French Re-Ligaire: ‘To bind back’]. We aren’t so smart to have newly awoken to it for the first time.
The Rig Veda is solemn invocation meant for performance at formal sacrifice. The Upanishad is mystical poetry in which teachings can be gleamed. Sūtra is fiercely focused, if cryptic, teaching. And so on. They each yield very differently to interpretation and translation.
The entire evolution and its settling was complete by about 500 BCE. In the centuries following, Shūnyam gets morphed into a rarefied space of high-abstractions and elevated reifications, all proxying for a missed denouement. [The Posts list about 40 examples.] The regression once again begins.
The Posts on Shūnyam in the main column, the ‘What, Where, How and Why’, while drawing on the historical trail, are defensible and complete on their own merit.
In other words, you don’t need to untangle its historical evolution in order to buttress the very present truth of Shūnyam.