Sometimes it helps to keep things simple
Dude, the explanation doesn't get simpler than 'a marker of xyz.' This is another reason why people shouldn't get too bogged down in grammar explanations when learning a language. I'm hassling about this because I'm actually quite interested in finding out why myself...
Why and how does the word się move in a sentence when accompanied by other words?
Good question. I think it moves after modal expressions like -
Muszisz się uczyć!
Probably other things too. I'm buggered if I know why.