(1) Each Virtualmin server has its own unique number, the sub servers also have their own number
(2) Each “pool” you see is a fpm server child process and the amount of these is according to the PHP FPM config in Virtualmin. Each child process is using the shown amount of memory.
(3) According to what is configured as “pm.max_requests” in the config
Each website has a user, and everything runs as that user.
ID is not a user ID, it is the process ID (pid). There will always be several PHP-FPM processes, in a default configuration. That’s normal. It is difficult to say exactly how much memory they are using, as most of the memory used by a process will be shared by other identical processes. The application code being run is, of course, different, and so is the data for each running process, but it’s hard to say how much that is with precision, without digging down into a specific process.
I don’t know what you mean by this question. I think maybe you’re confusing PID with UID, though (each process is not a user, though each user will have several processes).