Im trying to understand stalemates but i can't

That is not true. The stalemate is a universal chess rule, and it is not unique to chess.com. Stalemate on chess.com is the same as a stalemate in an over the board chess tournament. It is the same on every other website as well. The rule itself exists for a really long time. Here you go, if you wish to know more:
https://en.wikipedia.org/wiki/Stalemate
Yeah, prep them for the advanced stuff https://youtu.be/FkBNgYmdjRw