CAD Software to design 3D objects.
STL files.
Caliper to measure tools in real life and add dimensions
You draw and then extrude. When made in 2D you can extrude differently to create different areas. You have some options to make round edges and other things. That’s one way of doing it.
You can start from box object and then add other boxes or extract them to make wholes.
Slicing Software