Solid or shallow?

Hi! I’ve started quite recently with blender and don’t plan to become guru of it, just making basic models which I want to print.

The only basic question I have and don’t have answer yet - how to create solid objects? I mean 3d printer will make solid object, not shallow one with empty space inside.

For example I can build a cube with six facets. As I understand this will be just skeleton. But what if I extrude square into a cube? Will it be the same skeleton or solid object?

The extruded square will end up having 6 faces and an empty space inside; not solid.

It is still solid, if its manifold. All objects (faces) without thickness are solid if they have no nonmanifold elements.
If you want hallow object then add thickness, and that thickness part is solid part.