Part of a redline I did today (of Spiderwoman with a massive ass and muscles nothing less), turned out some people didn't know this so I am posting it
it is the smallest tutorial ever and yes it is drawn very exaggerated

and one thing I should probably say is if your arm is held out like on the right, but fairly relaxed and without effort, the muscle can tend to hang and flap a bit
This doesn't happen if the arm is stretched "in action", just the vertical thing without effort. On normal people that are not fat, have extra skin or are beefy it pretty much looks the same honestly.

Both those muscles draw up towards the shoulder when contracted, so they change shape quite a lot.

Related to this but not shown; a lot of people here and elsewhere draw godawfully short deltoid muscles (the one cupping the shoulder). They look shorter on muscular people because of the way they bulge, but really they reach halfway down the upper arm and that shows as a softer contour on normal people. It's also essential to think of them as that, cupping, since that means they change angle with the arm's movement and connect to the chest and back too. It needs leverage on the arm.
Just look at yourself if you're in doubt, I am confident 99% of you have shoulders ok