Based on my personal experience, big-corp monorepos have all features of a black hole: they try to suck in all the existing code (vendor it) and once some code starts living in a monorepo, there is no way to separate it as it becomes entangled with the entire thing. There is no way to "blend it in", so to say.
This subject deserves a study of its own, but big-big-tech tends to use other things than git.
This subject deserves a study of its own, but big-big-tech tends to use other things than git.