Best Way to Nest Collections

Looking for the best way to nest collection (or use some other method).

I need some way of relating CollectionA -> CollectionB.

CollectionA is a group of CollectionB. CollectionB should hold items that are grouped by some field in CollectionA.

I'm trying to generate a dropdown on the backend that a user can select a group from CollectionA, which will add CollectionB items to CollectionA's respective field item key.

Sorry if the explanation is hard to understand, let me know if you have further questions.