the biceps muscle is made of two heads, long head and short head. the short head is the medial part and is close to your body and the long head is the lateral part and is on the right side of your short head. the short head is more responsible for the width of your biceps and the long head is responsible for the peak and circumference of your arm. there are three functions that your biceps perform, elbow flexion, shoulder flexion, and hand supination. also, there are three dimensions regarding your arm positioning that affect your performance and which muscles and even heads of biceps you use. internal and external rotation, degree of abduction/adduction, and degree or shoulder flexion/extension. generally from a neutral point, the more internal rotation, shoulder flexion, or shoulder abduction you have, you are going to target more the long head and the opposite is true for the short head but keep in mind this is only true in case you do not mix them up and change only one dimension and keep the other two neutral. also, make sure you do not exaggerate the degree as well. also the diameter of the dumbbell or cable handle matters, the wider it is, the more emphasis on forearm muscles and brachialis and brachioradialis(which sometimes are also considered forearm muscles as well).