I know how to do this with selected polygons. But if I outline the edges and press P, it just creates an outline or something.

Answer 1, authority 100%

With P , only those elements that are selected are stripped off. If only the edges are selected, then only they will be separated.

If you want to separate a region inside, you must first select it. There is a function for this: Select – & gt; Select loops – & gt; Select Loop Inner-Region