I don't work at google but have many friends that do. (so take this with a grain of salt)

But just because they may put all their code and commits in a giant common repository, does not mean that its hosted, scaled, and deployed as a monolith in current day.

From the grapevine, I know that some of their stack is c++, and some is java, as an example.

Also, google losing all their code, made me laugh. Maybe that is a possibility, but seems like they have to have so many backups and failsafes. But I get you said that to illustrate a point :)