Let me explain how I understand csi-rs reference signals.
Each csi-rs reference signal is transmitted from one antenna port 3000s.
So if Ue has 3 pdsch layers with 8 csi-rs ports then there will be 8 csi-rs reference signals and UE will use 3 antenna to receive the pdsch layer.
Accordingly the PMI matrix will be exactly what UE measures with 3 antennas, each of the 3 antenna measuring 8 csi-rs referrence signals.
So PMIL will be LXP where L is the pdsch layers and P is the csi-rs ports.
So UE will measure port0 with 4 antenna, port1 with 4 antenna, port 2 with 4 antenna and so on.
So in our case of 3 pdsch layers and 8 csi-rs ports UE will report a PMI that is 8x3=24 elements inside it.
Ideea is that PMI cannot have any value (it woudl be too complicated) so 3gpp limited the values of PMI to a precoded book.
So UE has to chose from a set of predefined PMIs and chose the best one that fits him.