How are things like a USB casing designed for 3D printing?
3D printing provides a faster method for prototyping and have always been labeled as prototyping machines. Until recently, it has been rare to see 3D printers used for "mass manufacturing".

Yes, most mass-produced products start the manufacturing process with a 3D model. 3D models can be created in many different applications such as Solidworks, AutoCAD, Unigraphics, Blender, even Sketchup just to name a few.

In product development, the 3D model will then go through prototyping. Rapid prototyping can be done using a 3D printer by utilizing cheap materials and almost no labor cost. 

Here are a few costs that can be associated with the different prototyping methods.

Traditional Prototyping

(Typically involving "traditional", subtractive manufacturing methods such as CNC mills, lathes, routers, etc.)


CAM programming
Fixturing
CNC Machine Setup
CNC Operating/Labor


Rapid Prototyping

(Typically involving a 3D printer or other additive manufacturing methods)


Model preparation (for slicing)
Printing Operation
Object post-processing


Removal of supports
Curing/Cleaning of part (for non-FDM/FFF printing methods)



Once a prototype is produced, the designer will adjust the 3D model accordingly based on results of the prototype. This process will be repeated until the prototype is adequate for the purpose of the end product.

When the product design is ready for mass production, it will go through traditional manufacturing methods such as: 


CNC Machining (subtractive)


Mill
Lathe
Router
Laser
etc.

Injection Molding
etc.