The assembly creator lets you select whatever parts collection is needed for whatever the assmbly is used for. You can have lots of assemblies. Note the screen shot; there is a checkbox "Make Available for Doors/Windows" that will flag this assembly to be selectable when you put a door or window in. That's because an assembly might be 4 or 5 parts that create a special lock kit or inner framing.
When do you create an assembly? In part the answer depends on your parts situation. e.g. if you buy doors that come with lock kits then you won't need to add one. But if you buy doors that are bare, you need a way of specifying what goes with the door.
Here's a snap of the utility. Using it is fairly straightforward since you just follow
the 1-5 steps. Rinse lather repeat until you have everything you want in the assembly.
When assemblies are flagged to be usable for doors, they show up in the AVAILABLE SUBASSEMBLY KITS list when you are installing openings: