HI! You're doing good so far and welcome to the forums!
the armpit area is definitely one of the hardest areas to draw, but with a little more understanding, it can become a lot easier to digest! Before anything, I would like to mention that I'm still studying anatomy on term 3 as well, so some of the information I give here may be incorrect! I'm pretty sure most of it is correct though, so I hope this helps you!
Scapula
First off, I'd like to talk more about the scapula cause this is something I learned recently that may be important in understanding the armpit muscle!
If you look at my drawing above, you will see that the scapula rotates as it goes up, but it also slides out more. The scapula is not a bone that is fixed in place. It slides and moves along the rib cage to some extent.
Because of this, you may be able to see the muscles attached onto the scapula in some angles even though you're not looking at the person's back. For example, if they're twisting.
Importance of Teres Major
If you look above you will see that the Infraspinatus and the Teres Minor both attach somewhere near the shoulder area of the Humerus. However, the Teres Major slides under and attaches more below and to the side / front of the Humerus.
This is important cause this means the Teres Major will be visible when looking at the armpit.
Importance of Coracobrachialis
If you look above you will see that I painted over your reference and color coded it for you. The two overlooked muscles when drawing the armpits are the Teres Major and the Coracobrachialis. The Coracobrachialis is the muscle that will be visible in most poses where the arm is raised.
As you can see from this paintover I did above, the two muscles you were having trouble with were the Teres Major and the Coracobrachialis.
Simplifying Teres Major
The Teres Major in most cases can be blended in with the Latissimus Dorsi. If you look at my drawing above, you can see that I have blended the Teres Major and the Latissimus Dorsi together.
Simplifying Coracobrachialis
The Coracobrachialis in most cases can be blended in with the Biceps. If you look at my drawing above, you can see that I have blended the Coracobrachialis and the Biceps together.
I hope this has helped!
