> Yes cavity is inside but not very easy to select something after that :)

When you create a cavity in this manner (through shell or by boolean), then the cavity becomes just one portion of the entire object, so it is selected if the entire object is selected. So for instance if you want to do a boolean with the object that contains the cavity with another object, you don't have to do anything special to select the inside parts, it's all one object.

But it can be difficult if you want to do something that operates on a single face of the cavity, that's when you'll have to hide some of the outer faces.

- Michael