What muscles are used in a arm wrestle? Are there any exercises that help arm wrestle specifically? And I've seen videos of people snapping up their bones in an arm wrestle. How does that happen?

Believe it or not but arm wrestling is more a matter of leverage than strength.  The champions use their chest, core, legs, and arms to win. The ones who lose and end up rupturing a bicep or breaking an arm are the ones forced into a curling position - as if they were doing dumbbell curls.  Winners have their whole body leaned into and on top of the other person's arm forcing it down.