Shoulder pain isn't humerus.
Our shoulder does such amazing work! It allows such freedom of motion that we can do so many different things ranging from your favorite sport all the way to allowing us to perform our day-to-day activities (reaching into high cabinets, scrubbing the back, kartwheels...). It is able to do this because it has such a great range of mobility, but this leaves it open to instability.
We must also cover some anatomy here to lay the groundwork. Most of us (unless you are anatomy geeks like we are) see the shoulder as just the shoulder and aren't considering all the components. So let's take a moment to differentiate the two joints of the shoulder so that we can have a better understanding of how they function together. There are two joints that make up the shoulder (there is a 3rd "joint" that we won't discuss today, the scapulothoracic, but this isn't a bony joint so we can visit this another day)! The main joint of the shoulder is the Glenohumeral joint. which allows us to move the arm into flexion, abduction, adduction, and extension (as seen below). Therefore, the shoulder derives a good portion of its stability from this area.
and our shoulder(or pectoral) girdle
It is important to be able to dissociate them from one another as well as know how they work together. If you take a look at THIS picture:
You can see where the scapulothorcic joint is.
Below here are the muscles that connect and help with the movement of our shoulder!
You can see how most of our shoulder muscles attach to our neck and mid back. Collectively, all of this makes the shoulder a pretty complex joint with several options for shoulder pain or dysfunction to begin. Some of the most common sources typically come from the joint itself with arthritis developments due overuse or long periods of weakness/instability. Another large contributor is poor posture making scapular motions more difficult and binding to the shoulder. The last, but far from least, cause is poor patterning. This means we move in an odd way for a time wether it be habit or compensation or due to trauma that we never pinned to the shoulder specifically. This "odd movement" can lead to scapular dyskinesis -which is a fancy way of saying the wrong muscle may be contracting or relaxing at the wrong time during a movement pattern such as raising your arm or pushing yourself out of a chair or maybe you only feel pain with bench press or overhead pressing.
**disclaimer this is not medical advice, if you find you have pain or have injured yourself go to a licensed practitioner.**
If we're looking at how to strengthen the shoulders, there are a lot of moving parts and contributing muscles as you can see now! We need good shoulder motion, good scapular motion, adequate clavicle movement, and good thoracic extension and overall posture... whew thats a lot!!
Training rotation and flexion/extension movements from your trunk can help improve the movement from your shoulder. If we are suffering from loss of motion in the trunk, the shoulder can compensate and begin to feel tired or fatigued.
To strengthen your shoulders think of the basic movements that happen specifically at the Glenohumeral joint (the shoulder ball and socket) and then look at the movements our scapula (or shoulder blade) does!
Always think of your cervical posture as well during shoulder strengthening movements, keeping the neck forward or strained can affect the shoulder and take some of the stability in the shoulder affecting our shoulder movements.
Credit for pictures:
https://www.arthritis-health.com/glossary/scapulothoracic-joint
https://www.orthobullets.com/shoulder-and-elbow/3035/scapulothoracic-joint
Anatomy book