how are stickman's shoulders and waist look like when they are rotated towards the camera?

In all of the art tutorials I've seen the example of using a stickman has always been a problem for me, it is all ok when the body is just presenting towards the camera, but from a different angle? It's ok when they have to display a simple example to the viewers that never heard of figure studies, but they never do it to a pose like this. How am I supposed to draw that?

Well, your solution IS quite accurate. As is your observation, that youtube tutorials tend to use references that make it easy to apply the technique the makers want to demonstrate.

I think the answer is, if you are most interested in a perspective depiction of, in this example, a hip that is almost vertical to your point of view, then the stickman method isn't the perfect tool. You CAN still use it, and I actually think you got the most out of it in that instance, it just won't produce a very satisfying result (as witnessed by the fact, that you express your dissatisfaction). Tools have limits, and you can push those limits a bit with a lot of practice, but you can't make them disappear.

You are "supposed to draw" it to best encapsulate your desired artistic effect, and if one tool doesn't cut it, switch to another tool.

The problem with perspective is you need to understand anatomy a little bit. No, you don't need to memorize muscle names. You need the general idea of joints and bones and body fat.
After you know how a box bends in perspective, you need to apply that knowledge to the body; and to do that you need a general idea of where things go.

Aunt Herbert is very right. But you can still use lines and curves to guide yourself; you just need to be able to bend, twist and adjust the guidelines to follow the body.
I've attached photos of example guidelines that could be drawn when drawing this pose down below. With some notes of perspective:
https://imgur.com/a/CXt4Gwg

I'm not sure my guidelines would help you because I don't know how much your understanding of 3d objects and the human body is :,)
But good luck anyways!