I'm not a illustrative artist but I have studied visual thinking heavily and I think you need some friendly objective criticism.
@Environment WIP
Bold lines have become the focal points, primarily by nature a viewer will look at the bottom left characters' faces especially the most active one (further away) and the character emphasised in the distance as well as the path (which kind of looks like a carpet?) leads the viewers' eyes around. This is good.
The perspective is a bit off, the foreground feels like it is curving downwards on the left bottom and it feels like you're using the bold to emphasise the focal point and when complete these areas may not become so much of a focal point anymore even when detail is added as the visual weight of the emphasised (bold) stack will send the right side out of balance leading the eye out of the picture through the top left.
@For a friend
The style strikes immediately, this is good and gives the piece a strong sense of character and the colours compliment each other well.
The two things either side the character (stylised trees / hand sculptures?) are near enough the same colour as the character and the negative space is leading the eye out and own away from the character which is not a good balance, the character is lacking quite a lot of rhythm in the body (left to right motion) like the character is floating and one hip should be higher considering the pose and composition to give a better asymmetrical balance, the left arm seems to cut behind the character at the joint which looks strange like a over-exaggerated armpit, try to work on your anatomy for this kind of character a bit better in future, practicing front-view poses using the gesture drawing technique.
The details here are very well contained, shape-repetition in the top part works well though shape repetition could also be added to the lower half, the eyes and the top part of the pose really emphasise a sense of focus on the character's face.
@Life study
Judging from the lighting etc I would assume you did this on a computer like the previous and you have contained the colours very well though I cannot give a full critique without knowing the original reference(s).
The shapes and their perspectives are a bit off, you perhaps were moving your head up and down which distorted this, though I figure you may have been looking down slightly more than the lower part of the can is conveying and in that case, make sure the perspective of the lower horizontal oval is in proper perspective relative to the top oval and force the shading to not look flat at the bottom as it destroy the sense of dimension. I won't consider it an error per se but you didn't finish the bottom right of the object beneath the can which can make your pictures open to visual distortion when reviewing from all of the graphic within the frame at a certain focus of the elements.
You have quite a good sense of realism considering this looks like a study focused on the overall or a few elements, though I would recommend you try studying form using a line drawing and make that accurate line-wise and paint over that considering what seems to be your natural sense of visual thinking (detail, emphasis).