>>953To put it simply, you make a copy of your base model and delete all of the bones/physics/rigid joints. Anything that's not the mesh basically.
Then you add your clothes or hair on top, fit and resize them. If there's, say, breasts poking out of the shirt, you don't need to bother with it right now, the main point is that it generally fits the body.
After this, you delete all of the body mesh from the material folder, leaving only your fitted clothes/hair. Then you save that model asnew and open your original model, then drag and drop your fitted clothes/hair on top, it's going to ask you if you want to open as new or "add", pick add, it will automatically merge the bones for you, so the clothes or hair move together with the body.
About earlier: if there's a bit of a body showing through, you usually just deal with it by deleting vertex of the parts that are poking through, so in that earlier example finding material for the torso and deleting the boobs.