Convert to solid

I've extruded a section I made of a sofa chair, how do I turn it into a 3d solid so I can subtract/union it with other objects.

Thicken?

See the tutorials in my signature.

That works but I wonder if I can do it another way without 'thickening' the extruded surface

few different ways, hatch, actually do it via the solid way then choose what ever shape you want, fill faces, and as you said thicken.