Does it not have an MMU and cannot implement processes, but only threads?
You are basically correct.
Some implementations have MPU, but there are none with MMU.
https://developer.arm.com/documentation/107565/0101/Memory-protection/Memory-Protection-Unit
Thanks, but why is the name process stack and not thread stack?
I think 'process' is a more general term than 'thread', though I do not know if that is the reason.
Well, thank you for your answer.