do You talk about an assembly drawing (2D) or an assembly 3D file?
And if it is a 3D, what are Your targets?
Should the new assembly be a subassembly of the existing one - what is a simple task - or should all parts be equal members within the existing one - what also is no difficult task.
If so, then first add the subassembly to the exisitng one and then use "move parts" to bring them into the top assembly in one additional step.
Drag and drop it in........
I use explorer to drag and drop everything in.
The only times I don't use that for file management, I'm either creating an assembly or draft from an existing file, or using revision manager to copy/relocate file.
A few ways to look at this.
If want to insert a copy of a part, leaving the part where it is, and use it for multiple models, just drag and drop it in from wherever that part lives.
If you want a new copy of the part for modification, I copy it using windows into the same directory as the assembly, then drag and drop it in.
If you want a new copy of the part as you copy the entire assembly, that's part of the revision manager software.
Also worth note: I work in what is called an "un-managed" environment. I don't use insight or other 3rd party software.
how or where do i find the "move parts" function to bring them into the top assembly?
I think you need the "transfer" or "disperse" command. Simply type one of the words in command finder when you're in the assembly. But they are in the home tab under modify next to replace part.