Thank you very much for your reply! I'm using a translator so I may have misunderstood in some places, I'm very sorry. What you said about comparing the reference picture to the lineart is very helpful, and the critique above the picture is also very good, they visually show the reason why my drawing doesn't have three dimensionality (I think the reason is the Pectoralis major and the protrusion of the ribs on the frontal side of the abdomen).
My understanding of the second point you made is to understand people as a combination of several geometries when drawing them, especially pictures like this one with occlusion relationships and perspective
I tried to modify
A version with shadows added (but not more 3D)