(Though honestly, the value from the internal code search was that everything was in there. So if you got an error message from a service you were a client of, you could just search for the error message in their code and see what it actually meant. "Aha, this 'optional' field that's marked 'deprecated' isn't actually optional or deprecated," most of the time ;)
The code search functionality is the same as what Google engineers use internally. Cloud Source Repositories uses the same indexing and retrieval technologies on the same type of infrastructure.
As you mention, the more code you have, the more benefit you get from having fast search tools across your entire code base that can perform complex semantic and regular expression queries. Even with smaller codebases, I find it the fastest way to find the code I need.
(Disclaimer: I work at Google and am the PM on this product)
Correct. Looks like it just parses and indexes the source.