It's confusing, I agree. It seems that there has to be the one display available to act as a base for new products to be added. In the basic sample store, it's called "Product display." In the full sample store, there is nothing called "Product display," but there are many product displays (Hats, Bags, etc.) but no way of knowing how to create them.
I managed to create a new product display once somehow, but couldn't remember how I did it! The confusion is compounded by the fact that Product types are a type of Content types.
The difference is explained in the documentation, but I'm still trying to figure out how to get the Add a product screen to display the common fields I want with the variations revealed beneath. I know it's something to do with the various Content and Product type fields, but it isn't clear to me as to how to make this work.
I think that the bottom line is that Drupal Commerce 2.0-rc1 is still very much a candidate release that needs work to make it useable for ordinary human users who aren't Drupal developers. Oh well, the bleeding edge....