The shoulder complex is the most mobile of all joints. You can lift your arm forward, to the side and back, and can also make big circles. Since it is involved in almost every bodybuilding component and movements, it might require a little more creativity to hit them well and get the rewards of “boulder like” shoulders.
To get the rounded shoulder look, the goal is to target mostly the medial (top of shoulder blade) and posterior head of the shoulder (spine of scapula) and all inserts on the deltoid tuberosity of the humerus (long bone of the upper arm).
The anterior head (lateral portion of the collarbone) gets a lot of work coming from all kinds of presses you should already be doing for chest and triceps.
Here is a way to bring out the lateral head, using specific strength curves by adjusting the angle of the arm to the body, to target the origin (A1), the muscle belly (A2) and the insertion (A3).
A1 One arm, lean away lateral raise low pulley x8, tempo 4010, no rest
A2 One arm, standing DB lateral raise x8, tempo 4010, no rest
A3 One arm, standing low pulley lateral raise x15, tempo 4010, no rest
Do the other arm, then rest 60 seconds, 4 sets of each arm.
You might want to think about washing your hair a little later after this one.