Google did not design a VCS from scratch, they adopted Perforce and started replacing its backend as they hit scaling limits. They eventually added some kind of

Google did not design a VCS from scratch, they adopted Perforce and started replacing its backend as they hit scaling limits. They eventually added some kind of copy-on-write FUSE layer that looks like your workspace contains the entire monorepo, which was pretty cool. It would be hard to open source if its dependencies only exist on Borg. I think that was a problem with Blaze (now Bazel).