PMI in CSI Reporting and Hybrid Beamforming

Hi experts, I am having trouble understanding the relationship between PMI in CSI reporting and Hybrid Beamforming. So after transmission, the BS will get the CSI-RS feedback report from UE and it contains PMI. And suppose the antenna elements are larger than 32 and I have 32 or fewer ports. Then there is antenna-port mapping related and I suppose it’s a hybrid beamforming question(not quite sure). The hybrid beamforming matrix is supposed to be y=HF_RFF_BBs+z; where F_RF is analog beamforming matrix and F_BB is digital beamforming matrix, then F_BB is the matrix of N_tRFN_layers and F_RF is the matrix of N_tN_tRF. But I looked up at the CSI-RS codebook and the precoding matrix has the dimension of csiportslayers, which is the same dimension as the digital beamforming matrix. So my question is if PMI only tells BS how to design F_BB, how do we do with F_RF. Or maybe I understand the whole thing wrong, then how do we use PMI in hybrid beamforming? Thanks a lot!