The workbench used to create assemblies of mechanical parts is called :
Assembly3, for the creation of conventional assemblies (or subassemblies).
My personal experience leads me to think that there are at least two different situations depending on our need :
1- The project is a set in which all the parts are completely interconnected :
Solution 1 :
Parts can be built in a MultiBody file of PartDesign in which they are represented in different Bodies. In this case the bodies can be drawn relative to each other using ShapeBinder.
Solution 2 :
Parts can be built in separate files. In this case, an Assembly WorkBench is welcome. It allows to put in relative position the different objects in order to realize the complete product.
Note that I do not voluntarily talk about sketches positions, objects, or others, which assume numerically known the desired positions as well as the dimensions of the objects.
This is not always possible in team work, or even when you use older components in a new project whose details are no longer known (also true for standard elements from component libraries).
2- The project is a mechanism in which certain parts retain the mobility necessary for their work.
It is then composed of classes of kinematic equivalence, that is to say groups of parts completely related to each other. These groups are subassemblies that will be made according to the solutions 1 and 2 above.
A general assembly will collect these kinematic equivalence classes, and the Assembly WorkBench will create the specific kinematic connections (joints) needed for the assembly to work.
This assembly can be used in simulation programs of kinematic or dynamic behavior of technical solutions before proceeding to real tests on prototypes.