a push to a repo doesn’t mean “deploy changes to everything” or “build everything” any more
What do you mean? I have yet to work for a company that's organised and sophisticated enough to actually use a monorepo but my understanding is you'd set up something like Bazel so it only builds & tests (and I guess deploys) things that depend on your change.
I don't think many of us need to worry about scaling beyond the size of Google or Microsoft.