So much of drawing the body with irl proportions is based on proximity and reference points
The more of these little mental notes you can cram the more you'll understand the what and why of the proportions of the body
In this example you can see how the width of the clavicle is roughly the same as that of the lower ribs, and this is where the arms begin. When adding muscle, the arm's additional width comes from the packing of shoulder tissue onto the humerus bone
Know your bones