The glenohumeral joint is an extremely mobile joint who’s stability come from numerous soft tissue structures such as ligaments and muscles rather than closely fitting joint surfaces as seen in the acetabulum/hip joint.

Although this dynamic stability is very complex in reality, an easy way to grasp the concept is to think of the joint as a series of concentric circles with the glenoid fossa and labrum at the centre and the rotator cuff muscles and prime movers at the outsides.

Many thanks to Carl from Neo Health for showing me this.