Google's monorepo includes snapshots of everything upstream that Google's proprietary code depends on - starting from the kernel and glibc and core language toolchains, and all the way up to various open-source libraries and modules in c/python/java/whatever.

But 3p sources likely are for specific stable version? Linus can't break Google by submitting bug into linux kernel?

Yes.

Chrome has brought in third party code. Which makes Debian mad and they do a lot of work to strip that out to use the upstream libraries. (Or at least that is how I understand it - I don't follow Debian closely)

Yeah, that's why I wrote "mostly" :)