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.

That black hole behavior is a result of corporate processes though.

Not a result of git.

Business continuity (no uncontrolled external dependencies) and corporate security teams wanting to be able to scan everything. Also wanting to update everyone's dependencies when they backport something.

Once you got those requirements, most of the benefits of multi-repo / roundtripping over releases just don't hold anymore.

The entanglement can be stronger, but if teams build clean APIs it's no harder than removing it from a cluster of individual repositories. That might be a pretty load bearing if though.

> there is no way to separate it

There is

git subtree --help

May get complicated at the edges, as files move across boundaries. It's a hard problem. And for some subset of those problems, subtree does give a way to split and join.