Does sketchup free have any kind of automatic shelling or hollowing feature?
That being said, this question is most often asked by people who are preparing to 3D print a model. If that is the case for you, then you don’t need it in SketchUp! Why? You’ll be exporting to a .stl file, then use a slicer program to generate the gcode your 3D printer needs.
And every slicer program (that I’ve seen or read about) will do it for you. Look for “Wall Thickness” and “Infill” settings. The “Wall Thickness” tells the slicer how thick you want the outer boundaries to be, and the “Infill” tells it how to fill inside it. You usually need some % of fill in order to support to the top of your 3D shape.