Basically what I’m after is this.
How do I cut off a piece of a larger object, and save that as a seperate object.
In Edit Mode, select the part of the object you want to separate, then press P and choose “Selected” , go in Object Mode and you can now select this part as a separated object.
can I save it as a seperate .Blend file as well?
So many ways to skin this cat, but I think your method is by far the simplest. I always duplicated the object and cut off the excess for the longest time till learning this. 
If you wish to save as a seperate file just delete one or the other of the objects, then save. then ctrl z and then repeat for other part.
okay, so far so good.
How do I add it to a active file? I’m making a pistol and just need to add a trigger, hence the seperate part I just saved.