These files generally come in a stereolithography (STL) format, and you’ll need to load them into a slicing program like Cura to generate the actual G-code file that controls your printer.

A printer is essentially a sophisticated plotting machine that moves to certain coordinates told to it by the slicer in the programming language G-code. That's why slicer software is so important.

There’s one that gives you a simple drawing program for making your 2D image, and then produces G-Code so that your 3D printer can extrude the drawing as a single plastic layer.