Earlier quoted context omitted.
The benefits of monorepos are (IME) all in people and organisation scale. For example, I've found it orders of magnitude easier to align tooling, quality, release and standards on a monorepo with 100's of 1000's of engineers working on it simultaneously. The point you've made is a good one though, the idea of monorepos at small scale has come up a lot recently and I'm not sure the arguments for it are as compelling (…
Do they use git or some other versioning system?
A small, illustrative example of this is the tooling that's built for Chrome's source code[0] with the depot_tools project [1].
[0]: https://chromium.googlesource.com/chromium/src/+/master/docs...
[1]: https://chromium.googlesource.com/chromium/tools/depot_tools