The arms look awkward because:
1a) They feel like they don't have bones. Just like pool noodles.
1b) The wrist is thicker than the biceps, no structure
1c) The lines of the forms are either almost parallel or are clashing with each other, so the result is stiff and unnatural
The solution would be to understand how bones and meat work. So study anatomy more. And dynamic drawing. Here are some basic pointers:
Here's the simplified bones of the arm, from the front, palms up
They don't change, they're always there and they bend and twist in pre-determined ways. The root of the whole arm has an impressive range of movements because of its round joint. Up, down, back, forth, all around - it can move in all the ways. Tho it does have restrictions, it can't go back very far thanks to the scapula
Elbow joint's deal isn't as good. This poor sucker can move only this way. No bending back
But Yon, how does the palm rotate around, you ask? It's thanks to these bastards
+shoulder joint
The blue bone follows the thumb and twists from the outer side into the inner side, hovering over the purple bone. The purple bone moves a bit too, but it's mostly working as a stable fundament for the blue bone. The shoulder joint twists a bit too for more comfort
The wrist joint is a bunch of nonsense
But the important part is its movement range. It can move up and down pretty well
But the side movements are limited
So we finished the bones. Now let's move to the squishy things. Muscles are the most changeable parts, they twist, bulge, squish, and tense. They work as pull and push system, so most often they come in pairs. When one of the muscles tenses and rounds, the other relaxes and stretches.
Here are VERY simplified muscles
Here how it looks from the side. For example sake, this arm is very beefy. The elbow joint isn't covered in much muscle because otherwise it'd be impossible to bend it. But the outer side of the joint is covered only in thin muscle head-stuff and skin, so it pretty much sticks out. The inner side of the arm is the place where the squishy things happen, mostly
The most meaty part of the elbow area is pink blob over the blue bone. It pretty much acts like the blue bone and follows the thumb.
The wrist doesn't have much muscle, so it's the thinnest part of the arm.
SO! Let's go back to the picture. Let's put some bones
Remember, the elbow bends in only way + the muscles are mostly in the inner side + the limited movement of the wrist
Et voila! Something like this, I tried not to change the pose too much, so it still might be wonky somewhere.
This took me more time than expected, so I'll talk about other points later, maybe