How to create this solid model?

The incomplete drawing is most likely just another poor 3D modeling /drafting course someone has paid good money to attend. 3D modeling in an engineering environment is based on adding or subtracting solids. All of these solids are produced by extruding, lofting or revolving 2D sketches drawn on various planes.
a place to start would be to sketch the base part on the X-Y plane then extrude this 35mm in the Z direction. Then build each part up from there. If you are new to engineering it may take you awhile to get the actual shape of the finished part in your head. Looking at a 2D drawing and picturing what the actual 3D part looks like is a skill that takes practice to develop. Get a pencil and paper and try to sketch what it looks like can sometimes help. Until you can picture what the object actually looks like in 3D, modeling it will be difficult

Added 3 images which outlines starting steps. Once you have the outer shape sorted cut away bits to make inside