I think all of those are using git and many Gerrit as well.

If your project is open source but still interacts with the main body of Google code in a significant way, you can still be in the monorepo and then mirror it to Git via Copybara (or you can work primarily in Git and then mirror to the monorepo via Copybara).

The most common case of that is libraries that get imported into server binaries. There's a whole section of the monorepo for open source code, with special rules and processes around it.

But a that has setup and friction costs so it's only worth it if you see a pretty significant upside.

For stuff that's pretty standalone there's no point. Classic cases are Chromium and Go. Then of course there's Google's various operating systems...