I haven't worked at google, but my understanding is that their monorepo does contain everything, including kernel and libc etc. So it's not bunch of random tarballs, its complete in-house maintained source tree.

> But then.. do they ship that glibc with the end result? Or just shrug and hope it works fine on whatever glibc the target system happens to have?

That's the whole point of monorepo, you don't have some random target systems, it's all included in the same repository.