I want to make the belly fat and ribs in my painting more three-dimensional

The head is a bit weird too, is it because of the proportions?
[img]https://ibb.co/RSP7pmH[/img]Reference pictures and my drawings
[img]https://ibb.co/1TMvWXR[/img]
[img]https://ibb.co/sCTZf5H[/img]
[img]https://ibb.co/x66ydSz[/img]

Just a heads up, those are links to ibb website pages. They can't be embedded. You need to put the link directly to the image file to use the img tag and have them embedded. These links would end with .png, .jpg, .gif or .webp. For example: https://i.ibb.co/PggR0Z1/MTXX-20241220-015036601.jpg

Which looks like this in the img tag:

Thanks! I tried it again

I think this is where it can be very helpful to both trace your lineart AND the reference image! That way you can see the difference between what you observed and what is actually present in the reference image very clearly. 

It would look something like this: 
[img]1000x600[/img]https://cdn.shopify.com/s/files/1/0781/1055/6489/files/chest.png[/img]

As you can see with these two tracings next to each other, to me there's two simple observations to make on why the chest looks flatter in your sketch.
One is that your sketch lines are rather straight and don't quite capture muscle curvature (which can be fine btw because sometimes that gets distracting, depends on where you want to put your focus).

The second (and more important) one is that you have, in a way, stretched out the body. Because are used to seeing humans straight on, even when we look at something that's as foreshortened as this image, we tend to want to draw it as if we see it head on. This is where you kind of have to fight your brain to be able to draw what's there, for me that looks like trying to no longer read the model as a human, but just as shape. 

As a more direct edit to the piece you made, you have to give some space to the shoulder muscle and ribcage. A lot of the lines you've drawn inside the torso should be up higher to communicate the width of the chest! 

Hope that helps you!