How do polygon hit boxes work??

I need help. How do polygon hit boxes work?? Does it set the hit box to be the pixel art size, because that would be extremely helpful. If not, there needs to be a way to set hit boxes to be smaller than 1 grid space (32 pixels). For instance, I’m messing around with making a bullet hell game, and I need bullets with SMALL HIT BOXES, and I don’t know if this will work. Also, how does a capsule hit box work? I am also unaware about that one.

Polygon fits to the size of the sprite by pixels
Capsules I think have no friction on the top or sides.

Thank you JR!