besottedgran I believe - but don't blame me if I am wrong - that this is how it works.
Just as a standard printer goes from side to side across the paper, putting little ink dots to form the letters and pictures, the 3D printer goes from side to side horizontally in a container of plastic powder, firing dots from a laser (or some such thing) to melt it together where the thing it is making is, but not where the gaps are. Then it moves up a tiny bit and does another layer. The computer has the blueprints for each layer. By the time it gets to the top, it has fixed together all the layers to make the 3D Thing.
Don't ask me what happens to the loose plastic powder that was outside the shape. It must fall off when the Thing is taken out, but what if there is a cavity? The spare powder would be sealed in for ever. Maybe the Thing has to be designed so as to not have any cavities.