I've also heard (not a Googler myself), that parts of the code are private, like very specific parts of the search algorithm that are very critical IP. So while a vast majority of the code is readable by most, it's not some kind of completely transparent place.
Yes, this is true. It's usually code with sensitive IP in it, which is limited to the people working on it. The API around that code and the binaries that result from it are exposed to other teams, but not the original code.
