Well, it's not that hard y'know, all these can be done with a little more practice, a little more observation, a little more studies, and the key element - by first understanding the skeletal structure of anything and everything that you're trying to draw in your manga.
Skeletal Structure Concept:
By skeletal structure, I'm not asking you to draw a skeleton in your manga. What I mean here is the basic frameworks or shapes that gives an object its outlook. For an example, a book can be drawn by first sketching out a rectangular cube, which is the skeletal structure of a book:
No matter it is an open book or a closed one, the skeletal structure remains the same, which is the rectangular cube. Meaning, I can draw anything in my manga as long as I've got the grasp over the basic shape of the object that I'm drawing.
So with the skeletal structure concept in mind, let's move on to drawing complex objects!