It's worth noting that this is only viable at Google because they don't use git. Git's insistence on every client having a full copy of all history of every file in the repository makes monorepo much more expensive.
I see conflicting reports over whether google use Perforce or something proprietary called "piper"?
You might be happy to know Microsoft created a Virtual File System for Git[1] [2] so you do not have to have every file checked out in your working directory. Microsoft uses Git in a monorepo (for Windows, and it's 2.5 million files/300GB[2])
(It may still be backwards-compatible, but it's been years since they turned off the last real Perforce and I haven't worked there for years myself. So it may have diverged.)
Note that there's nothing forbidding you from writing a virtual git filesystem that fetches objects from some centralized repo as files are open()ed. Git on cloud steroids.