Please change your .SVG diagram of the RPI-4 It mentions it has an 8GB SRAM version, which is NOT true, and probably never will be true. Rumors about an 8GB version were based on a printing error, and although there may have been early plans to try to bring out an 8GB version, its clear that nobody makes such an SD-RAM module, and probably nobody will.
So your picture is creating unneeded confusion.
also the rectangle you marked "POE control" is simply a decoupling cap to cope with USB rush-in-current. All POE logic is contained in the POE HAT, except that the Ethernet mag-jack is a special type for POE, and there are a few headers to connect the POE signals to the HAT.