[9dev]

It’s not. Google uses a content warehouse database internally that holds all stored web page content, and to access this vast database, they have an API. The code discovered here is a generated SDK for Elixir for this content warehouse API.

Apparently, Google had a now deprecated product (who would have guessed that? Consider me shocked!) that provided customers with a trimmed-down version of this database for their own purposes, but mistakenly published the internal SDK code instead of that intended for Google Cloud customers to GitHub.

So while this doesn’t directly show the search index source code, it describes the data schema of the index in great detail, so there are at least some interesting educated guesses on the workings of the actual index to draw from it.