Could you even put all of google3 on local disk if it were allowed?!? You'd need quite a RAID array. I suspect it'd be almost impossible in practice.

There's no reason to pull the entire repo just to build one project. Do you pull all of GitHub to your disk?

From the user interface perspective though, it does essentially look like you've pulled all of google3 into your disk.

Because of high coupling dependencies between google3 projects, compiling just a single project usually pulls hundreds of thousands of different build targets.