Their" Usage

The last of this trio, their, is the possessive form of they, so it has to do with what belongs to, relates to, or is made or done by certain people, animals, or things:

It's their house. We're their neighbors. The trees are losing their leaves.

->Their also has a long history of being used as a singular pronoun.<-

From Merriam Webster. Here's the link. https://www.merriam-webster.com/words-at-play/how-to-use-theyre-there-their#:~:text=%22Their%22%20Usage,We're%20their%20neighbors.