An industrial designer would sketch the outline then the images would be brought into parametric CAD software as references. The 2D image can be built into 2D/3D lines, then these curves are used to create surfaces which finally are stitched together. The nature of complex surfaces interacting and joining seamlessly is where the difficulty lies. "A class" surfacing is an entire speciality and can take a career to master. https://en.wikipedia.org/wiki/Class_A_surface