Some use the traps, some use the lats, some use the biceps, some transfer the weight early, some late, some not at all, some use the triceps to steady the bow arm, some just hold the bow arm out there, some use varying degrees of hand and forearm muscles, some use different combinations of all of the above. Some use core muscles to achieve different degrees of upper body rotation to take some of the weight off the upper body muscles and transfer it to the bones. There must be at least a thousand different ways to use different muscles in different combinations to draw the bow.