I haven’t worked at a Big Tech monorepo place, but I hope/assume that in the service dependency case, you would touch both in one changeset and the deploy tooling understands the dependency tree and orders the deployments correctly. Without that i agree its the polyrepo approach just with all PRs in the same repo instead of multiple.

In the library scenario, I know tooling like bazel ensures that’s one changeset, not 3+. Tests run for both the changed library and all consumers of it in the same pass. You’re right that it might loop in others for review who weren’t expecting it, but i think that’s the same in a polyrepo approach.