uuuuh im not the best at arms either but!! arms usually bend near the middle or bottom of the ribcage, and the wrist usually lines up with the crotch (with the hand hanging at or just below it)
in terms of actual shapes... it helps to kinda think of cylinders sorta? im no good with defining specific muscles BUT for simple arms a cylinder for the upper arm works, and then for the forearm its usually wider right where it connects at the elbow
the forearm usually tends to get thinner near the wrist, and theres also usually a little bump sticking out on like. the side of the wrist where the pinkie is if that makes sense?

none of this is set in stone and im probably the worst person to ask for drawing advice but yea lmao
it can also help you to observe how other artists tend to draw the arms, and references are always super helpful in general
idk if this was helpful but I Attempt

@elininjakat this is pretty helpful actually!! yea ive been looking t references and i just somehow Cannot get the forearms looking right for whatever reason lmao
i never really looked at where the parts of the arms lined up on the body i just knew it ended halfway down the thigh