you can protect the expression of the idea as reduced to a tangible medium.
In this regard, assum you have widget X. Widget X has ornamental features that can be protected by, for example, a design patent. The "design" of Widget X, such as blueprints, engineering schematics, drawings, pictures, etc., can be protected by copyright. Thus, someone cannot copy your blueprints without persmission. Or someone cannot publish the picture without permssion.
This is a very limited example.
When you say "phsyical product design" you kind of intermingle both of them, which doesn't leave a clear picture as to what you are saying.