Google does not use Git for its monorepo.
Afaik Googlers can use git client-side. The central repo is certainly not git though.
Also parts of Google, like Chrome, Android, and Go, do use git.
Or Microsoft (Windows is using Git).
With extensions to make it bearable for their code size.
Just as Facebook does for Hg: https://bitbucket.org/facebook/hg-experimental
Microsoft wrote a new filesystem driver for git because extending the code itself was too difficult...
On the other hand, Google seems to successfully use Git in the same way.