This is not usually true in monorepos or polyrepos, and is quite a dangerous practice that nobody should use and hasn’t got much at all to do with what type of repo you use.

I worked in a monorepo for a long time where you still had to deploy versioned artifacts. So when you makes changes to B, you still have to bump version IDs, pass deployment requirements and upload the new version of B to internal pypi or internal maven or internal artifactory, etc.

Then consumer app A needs to update its version of B, test out that it works and that, from app A’s point of view, it is ready and satisfied to opt-in to B’s new changes, and do build + deploy of its own to deploy A with upgraded B.

Doing this in a way where a successful merge to master (or equivalent notion) of a change for B is suddenly a de facto upgrade for all the consumers of B is insanely bad for so many reasons that I’m not even going to try to list them all. Monorepo or not, nobody should be doing that, that is bonkers, crazy town bad. It’s a similar order of magnitude of bad as naively checking credentials into version control.