The history of Google's relationship to version control is even more interesting than editors - it went from CVS in 1998 to Perforce (P4) in 2000, then gcheckout and g4 in ~2006, then OverlayFS was invented in 2008, git5 came out in 2009, CitC obsoleted OverlayFS in ~2012, Piper built this all into the VCS in ~2013-2014, while I was gone from 2014-2020 apparently we got hg and jujutsu frameworks, and then when I got back in 2020 you'd just check out a .blazeproject from your IDE and everything would magically work. Many of these started as 20% projects (I used to have lunch with the guy who invented OverlayFS; interesting character and one of the best programmers I knew) and then got folded into the "official" way of doing things once grassroot adoption showed the execs that this was how people really wanted to work.
Let's say there were 100M file in the monorepo (an underestimate). You obviously never want to do a git clone of that. But what if clone and checkout were free? That's what CITC did. Creating a new workspace took less than 1 second and got you a FUSE filesystem that looked like it had everything in the repo. But nothing was actually downloaded until you opened files. And your local changes were also stored in the service. And this was available to the CI machines. See where I am going. CI did not have to clone the repo and apply your branch. CI just had your changes available. If you were just testing your 10 files there was no cost to having 100M other source files that were unrelated to your project.
This solves many problems that git worktrees attempt to address, but 10 years earlier, at less local disk cost, and more performant. I miss that.
Git5 would copy some directories but builds would still fallback to files from the monorepo if you didn't track them. It was convenient for me since I could just grep and do fuzzy matching from my editor. Now I have to do some extra work to avoid grepping the entire monorepo. LLMs sometimes still try to grep the entire repo lol.
Now, you could use a perforace, mercurial, or jj interface and it works fine.