What are exactly bootstrap processes, auxiliary processes, standalone backends, normal backends(user sessions)?

Hi,

I've always had a hard time distinguishing various types of
processes/terms used in postgres. I look at the source code every time
to understand them, yet I don't feel satisfied with my understanding.
I request any hacker (having a better idea than me) to help me with
what each different process does and how they are different from each
other? Of course, I'm clear with normal backends (user sessions), bg
workers, but the others need a bit more understanding.

I appreciate the help.

Regards,
Bharath Rupireddy.