Isn't the Google codebase infamously a monorepo with billions of loc? That seems like it fits "scale of codebase complexity" as well as "web scale"
Yes, but much (if not most) of that codebase is in C++.
It's a monolithic repo, but not a monolithic codebase. It contains many many separate projects, libraries, and applications. There's a lot of code reuse, but it's not like you have to wrap your head around the whole thing.