I’ve also been confused on the second question for a long time, in my current understanding, one should have been able to choose a set of decompositions, and there should be a set of decompositions made up of all-core-aten/all-prims ops. While from this repo, it seems that the API of choosing the decompose-set is not currently avilible and the core_aten_decompositions() is not a pure core-aten decompose-set.
About the relation between PrimTorch and Decompostions: PrimTorch = Core Aten + Prims (APIs of Core Aten and Prims can be found here), so it is fair to say “aten ops will be decomposed to core-aten and/or prims” or “aten ops will be decomposed to PrimTorch”.