The ring/sphere world component descriptions as gravity plating/generator as if in effect you are collecting all the micellaneous material floating around the system to use for the constructed world. You have to just ignore the fact that there probably isn't enough mass floating around the system to construct such a shell, and disregard that the exsisting planets and asteroids are still there when you are done.
Once you get past those two leaps in logic, it's a small step to think you could then mine that collected material for resources.
