So I would say the most important thing is that the APIs these are using as in mlx5 DevX (essentially direct fw access) or ibverbs are exactly the same regardless if it's CPU or GPU talking to it. So with that in mind the source of rdma-core, DPDK, ucx etc may be the most elucidating when it comes to low level details.

For higher level patterns again the APIs are the same so anything building on libibverbs or aforementioned ucx etc are pretty compatible from a high level ideas perspective. If you are new to RDMA in general definitely start with raw verbs instead of using abstractions like MPI if you really want to build a good intuition and then move to MPI once you understand what it is doing for you.