Need help adding volume to my sketches!

Hi everyone, looking for some advice on how to make my figures look a bit more 3D. I'm not going for hyperrealism, just want to add more volumes to my figures.

Added an imgur link at the bottom for the picture, first time trying this so lmk if it's not working out.

This isn't a page I'm proud of, just pretty average which probably makes it a good starting point to get better. The two top figures are 5 minute sketches (I had lost a bit of focus when drawing them), and the bottom one is 10 minutes.

Shading is one of the things I'm sure I should focus on a bit more, only the bottom one has a bit of shading and it really isn't my best.

"Interlocking lines" (not sure how to say what I'm trying to convey), to show one body part is clearly in front of the other, is something else I find a bit lacking from my sketches.

Any ideas or advice on how to study these things? And importantly, any other things that stand out to you that I should be focusing on?

Tysm for any advice, appreciate you reading me:-).

https://imgur.com/a/COOlDB5

Frankly, your lines do actually decently indicate volume. Which makes it a bit hard to come up with the one silver bullet tip, that is guaranteed to blow you to the next level.

The big youtubers always push towards more simplification, although I must say, it drives a tear into my eyes to ignore for example Jorges neat muscle contours to achieve it. In case you need a reminder, here is a very short clip, that lists 3-D primitives for mannequinization:

I while ago proko had this idea to completely express a pose (sans limbs) with only a box for the hip, a box for the chest and a box for the head. Sounded silly at first to me, but when I tried it out it indeed gave me some more insights about the orientation of the masses. You might want to try it, too.

As far as I have understood the idea, the way this simplification stuff "adds volume" isn't about changing so much about the lines you are currently producing when drawing from reference (apart from simplifying them and thus losing a lot of detail), it is more a preparation to go beyond the reference and allow a basis for manipulation. Like in, draw a pose simplified enough that you can reproduce the same pose, but from a different angle. And that is actually painfully hard.