Checkouts are apparently instantaneous. Behind the scenes, there is a FUSE filesystem that faults in files by querying Google's cloud servers. So it does not require a significant amount of space (but does require fast access to Google's servers, which can be problematic).

Almost all engineers have access to the "complete system," which is really Google's server-side software. Other repos like Android have historically been more locked-down, but there's been some recent effort to open them up within Google.

Presumably if you tried to copy all of the files, you'd first be rate-limited, then get an access denied, and lastly a visit from corporate security. I wouldn't want to try.