You are correct
@HJ57.
I've added a few pictures here for further clarification.
These are roof rails that come attached to cars from the manufacturer but can also be drilled into the car later, however, the cost may not be worth it.
These are roof racks, that can be attached to your car's roof at a later stage too. As you can see, roof racks mostly come with clamps and fasteners which allow you to attach and detach the roof rack at any time.
This is a roof rack mounted on a roof rail. As roof racks come with clamps, these can be attached or detached whenever required.
A roof box mounted on a roof rack that has been attached to the roof using clamps.
These are roof bags, which can be mounted onto the roof of your car even without a roof rack or a roof rail. These come with straps that fasten the bag in place and serve the purpose of an overhead carrier.