So what you are referring to is called parent modeling… (some engineers despise this 
it basically mean you create all the parts in one file and each part is made as a individually body within that file (i.e do not merge solids) you can then save out the individual solid bodies to their own part files, which then can be brought into a assembly. Thus if you make changes to the file that has all the solid bodies (the parent model) it will update all parts that are connected to those solid bodies.
This can be great when you have a complex surface that spans across multiple part and you want to ensure they all update in as few mods as possible. but the main model can become large, overly complex and very unstable… not something you want to be working with down stream while getting ready for manufacturing files.