> but it is very unclear how to do it with a distributed repository

Versioning through branching and tagging, while having some drawbacks - at least the fact that you have to DO an operation and that this is not automatic - seem to solve this problem, and are not, in my eyes, a form of monorepo. You globally get more flexibility at the cost of a bit more repo management work.

If the problem is retrieving the right version automatically, externals or submodules should be able to solve this problem. If A and B have no clear dependency direction, a top level repo might help.