A fist is a hand tightly closed with the fingers and the thumb. The knuckle is one of the joints where the fingers bend or join the hand. Please see : https://dictionary.cambridge.org/dictionary/english/knuckle

When I clench my right hand, it forms a fist. When I form a fist, it is easy to see my knuckles. Sometimes the word "knuckle" means any joint in a hand. If so, my right hand has three knuckles on each finger and two on my thumb, for a total of fourteen. Sometimes the word "knuckle" means the joint where a finger meets the palm. If so, my right hand has four knuckles, one for each finger.

Your fist is your entire hand screwed up. The knuckle is the joint at the base of the fingers next to the top of the hand.

knuckle is any of the joints between the phalanges of the fingers while fist is the act of breaking wind. fist can be hand with the fingers clenched or curled inward.

A fist is your entire hand - it's what your hand is called when it's balled up as if to punch someone. A knuckle is one of the individual bones that make up the joints in your finger. There are other meanings for these things, but based on the context you've provided, I hope this makes sense :)