Just curious, do you use monorepos as well ?
No in the sense that we have several thousands repos. Yes in the sense that some of these repos frequently contain several packages, both apps and libraries, when they're closely related and owned by the same group.
The goal though is for repos to map to team ownership (a team should be able to own its stuff, as I mentioned above), and teams are a very malleable thing (they merge and split all the time). Having repos be very small helps a lot with that (and helps tools scale without needing to do anything special)
We follow an eventually consistent model. That is, we strive to have 1 version of dependencies for everything, but allow temporary deviation. Sometimes those deviations are less temporary, but usually for reasons that would have prevented upgrades altogether for everyone if we enforced a single version for everything.
The main benefits are around ownership (people can do whatever the hell they want to their repos) and tooling/scaling (whatever works for your tiny personal project will work for any of our apps without needing to scale them). Those are separate benefits from monolith app to micro apps (because again, you can do either with either model). You also have some minor benefits that would be easy to work around but are nice to get for free (eg: Github's default history is actually useful in the web UI)
The only tooling you need to manage poly repos is something to do pull requests at scale when something needs to happen everywhere. You can everything else (eg: build management scoped to individual projects) basically for free, you can always clone the entire repo without issues, etc etc etc.