The first set ( A,B,C) is dealing with flexing and the different shapes of the muscles and which muscles are more visible or standout
A.) is a straight arm stretch, muscle on the back of the arm is the tricep which I do believe straightens the arm.
As you can see when the arm is straight the tricep is more define.
The Bicep helps bend the arm which we see here is relaxed(sorta) and is straight
Your forearm muscles are also flexed when in this position.
B.) We see that the tricep is relaxed and the bicep is being flexed, notice how the bicep has shrunk(sorta lol)
And also that the back of the arm is a little more rounded out.
(Bear with me everyone first time doing this just hope it helps)
C.) Is more or less the same as B but the arm is raised to see the inside which you sometimes(depending on the build) more muscle on the inside of the arm.
But notice the biggest differences in B and C are that you see the inside everything else stay the same this also apples if combine A and C
Next section torso
Which I will do later
But to put it simply what I do is all my bodies are roughly based of these design and it's just a matter of adding and taking away.
Also recommend that you study actual people like right in front, get some male and female friends to pose or flex and just yeah