I wonder why nobody have made a good public monorepo offering similar to what Google have internally. Would probably be a hit at many companies since it fixes so many issues related to working in very large teams.
That said, there are some open source pieces to help. Facebook open sourced their mercurial stuffs so you can get version control at scale (and before then you just use perforce). Google open sourced bazel. Google open sourced some parts of the underlying infra behind code search, but not enough to really work properly. And of course lower level there's a plethora of reasonable db offerings, etc.
It would still require a lot of glue though.
I'd like a set of tightly coupled tool like that working outside of Google, but I guess it might be a just a dream, it is a bit too big of a project.
The version control is just the tip of the iceberg though, and is largely a solved problem: git or subversion, then perforce or straight to Facebook's mercurial stuff.
It's the other tooling that breaks on large enough mono repos that you have hard time with publicly. Searching your code takes awhile. Cross references don't go wide enough or take too long to generate. Builds take too long. Refactoring tools either take too long or don't support the repo of sufficiently large size.
Mostly this isn't a problem though, because few repos are actually large enough to cause real problems.
It really is not a small difference.