"google3: The internal name for our main Piper source repository, and identifies the third incarnation of the source layout for Google production code."

"Piper: Google’s internal source control system: https://research.google.com/pubs/pub45424.html" Which is a pretty interesting read

https://opensource.google.com/docs/glossary/

google3 is Google's monorepo. Most Google engineers now work within google3. Exceptions include Android and most of the Chrome ecosystem.

Probably an internal repository.

Their monorepo

I don't work at Google, but it's been referenced before. I believe it is a main part of the monorepo. It at least has a lot of front-end code in it.

In addition to what the other commenters said, I believe it is also the name of their web server executable which includes the core Google search code. Written mostly in C++ but also ships with some Python scripts and protobuf linking it to other services.

I don't work at Google, this is just what I heard.