I went back to the explanation video and this appears to be the same or very similar drawing he uses in the abs explanation section. Basically, he uses two shapes to indicate the abs, a cylinder at the top and wider box shape below the belly button. Keep in mind that he says your free to use whatever shapes you feel best fit the figure. So yes it's ultimately up to you as the artist. Stan is showing us one way to do this but you are free to take the principle and apply it as you see fit. Also, these mannequins are simplified versions of the form. For example, pelvis, joints and the head don't have actual box shapes. Boxes are used to help us understand plane changes and orientation. As for line weight, I would say use a lighter line for the gesture and a heavier line for the 3D forms like the abs. Lastly, since you said you are new to this course, make sure to watch previous lessons in this course. All of Stan's concepts build on each other.