A part, an assembly, and a drawing are three distinct stages and types of files used in engineering and CAD design
A part is a single, solid object
Eg: Bolt, nut, steel shaft
An assembly is a collection of multiple parts put together
Eg: Engine made up of piston,connecting rod & more.
A drawing is a 2D blueprint showing how to make or build them
Eg: Blueprint created from a part or an assembly