Nope, that's not quite how it worked. Except for a few critical libraries that were pinned to specific versions, every build built against HEAD. However, a presubmit hook would run tests on the transitive closure of code depending on the changed code, and if anything broke, it was your responsibility as the committer to fix it or roll it back.

This is really what kept things stable. If you maintained a a common library and wanted to update it, you had to either keep things backwards compatible, write an automated refactor across the codebase (which wasn't too hard because of awesome tooling), or you would get dozens of angry emails when everything broke. If you relied on a library, you had to be sure to write solid e2e tests or your code might silently break and it would mostly be your fault.

I definitely miss the big G, so many resources were dedicated to engineer productivity and tooling. It's what makes the monorepo work.