They/them is usually used for multiple people but now they replaces he or she. Them replaces him or her. But to be honest as an English speaker it still sounds so unnatural for me because I keep thinking its like multiple people. Unless someone specifies that they want to be called this it is better to use he/she or him/her.

Oh and their shows possesion like his or hers.

You can use they/theirs and them to replace him,her, she, he and he's also she's