These four items were made the same way, started with a vertical rectangle on top of the block, modified its shape and then push pulled it. My question, why do some shapes appear solid while the others are not?
If the block is a component all shapes are solid.
If the block is loose geometry SketchUp can see the Push/Pull operation on the connected upper bit in the same way as pushing a hole as for a window in a wall. When the block is wrapped up in the component container, it doesn’t impact the Push/Pull operation so then you’re just making a 3D shape. You could just delete the block and get the same result or a solid extrusion on the top piece.
This is the same sort of thing that happens when people draw a floor plan on a large face and extrude the walls upward. From underneath, the walls are open at the bottom.