Most orgs at that scale build their own tooling to make working with monorepos more ergonomic, in my experience. They're based on git (or other DVCS tools like Mercurial) but they allow you to easily do things like partial clones and company specific workflow tasks like submitting patches for review or tagging in weird and arcane ways.

A small, illustrative example of this is the tooling that's built for Chrome's source code[0] with the depot_tools project [1].

[0]: https://chromium.googlesource.com/chromium/src/+/master/docs...

[1]: https://chromium.googlesource.com/chromium/tools/depot_tools