The C64 Ram Expansion Unit reference is about how the external ram access the C64 memory, I believe, in a DMA style.
AS far as the Memory Maps are concerned, how did Commodore expand past the 68000 in to the 68020, etc. cpus.
Multiple Z2 and Z3 cards could have possibly more memory combined than what the 68000 could address.
If I remember correctly, there is Port Mapped and Memory Mapped I/O in a system.
Each Z2 card is held in suspension until it gets its own memory address.
I do not know how many bytes each card gets.