Why isn’t this checkmate?

Hi, this is a mistake many new players make. This is called a remis It you look closely, you can see that you actually don't have anybody playing on the opposite site. And if you don't have somebody to defeat/ be defeated by, it is considered a draw

Google en croissant

Honestly white should have just resigned, all it does is delay black from mating 2 moves later.

Because the queen is cheating with the other king

Maam down! There's a sni