[Well, that saves him the explanation, then. But there's probably one thing he doesn't know about it because it's not something really talked about.]

Successful human transmutation is impossible. When we're born, we're born incomplete. Conscious, but immobile and in horrific pain.

Keeping that in mind, it's fairly easy to see how she gets homunculi on her side. She works her magic-- takes the pain away, and she has loyal minions on her side.