There is a wonderful project called Visual6502, and I've used it from time to time to figure things out, like exactly the difference between
rts. It is good to see which control lines were fired in each cycle from the Control ROM.
But Z80 is a whole other thing. It's not microcoded like the 6502 is, but instead apparently contains some finite state machines and things. I am not sure I understand why that design choice was made, especially considering the relatively more complex ISA, including variable-length instruction codes. But I really have no idea about that.
So I am interested in finding out more about how the Z80 worked. Has this chip been reverse-engineered and had its internal mechanisms explained, such that it can be understood by laypeople?