What's the difference between a thread vs. a process?
MySQL has been implemented on a lot of Operating Systems -- From Atari to Windows to Solaris to Linux.  Different OSs work differently when it comes to "processes" versus "threads".  MySQL picked whichever was better in each case.  Then it got sloppy in saying "THREADS_runnng" and "show PROCESSlist".

So, in MySQL context, "threads" and "processes" are synonymous.

In Operating System context, they are different enough to necessitate multiple courses in Computer Science.

In MySQL, one connection uses one thread (or process).  There is also a main thread (process) for coordinating between the connections.  Also, InnoDB has some extra threads (which may actually be threads and not processes?) to deal with background I/O tasks.