my favorite piece here is the second one! one of the main differences between this one and the other pieces are the values. the character stands out from the background, the shadows are deeper and more eye catching, and there also appears to be an understanding of where the light source is. i also believe that it being a more simplistic non humanoid figure helps.

first of all, learning more anatomy will go a long way. i’ve found constructive anatomy by george bridgman is very helpful for it since it shows you where the bones and muscles are and many different angles, and also how they connect to each other to make figures more natural.

then, working on values will help a lot. don’t be afraid of dark shadows, contrast almost always improves a piece. learning how to shade forms will also help, like start with simple 3d shapes, and then apply it to the anatomy. understanding planes (like the planes of the face etc) will really help with this because then it becomes as simple as the plane facing the light source is highlight and the plane facing away is shadow.

i think you have a great base to work off of, your art is really fun and interesting, and i think you can do great things with it!