The 6501 was pin compatible with the 6800, but had its own instruction set. After the lawsuit, the pins changed and we got the 6502. This meant that if you enabled 6800 support on the Apple 1 you could drop in a 6501 and use it with software made for the 6502. If you were really ambitious you could use a 6800 but then you'd have to write a new monitor ROM. There's no evidence that this was done at the time, but about 10 years ago someone did it: https://www.youtube.com/watch?v=ag6pWUhps7U
The 6800 wasn't a drop-in replacement on the Apple 1. Installing one required some hardware modifications to the board. (It's unclear if anyone who owned one ever did this.)
Also Apple has used lots of CPU architectures over the years in various devices (e.g. time capsules, various dongles, etc).
Surely they must have RISC V deeply embedded in various locations invisible to the end user, both for cost and experience.
They could possibly even be used deeply in the bowels of the M and A devices doing some random housekeeping in the storage system or who knows what.
In a funny way the term “CPU” is returning to its mainframe roots with so many functional units harnessed into a system, yet at the same time having strayed so far from its original usage (with multicore, non-linear systems) as to have become meaningless.