At a high level, the 3D printing process includes slicing a CAD file into discrete layers and then building that part layer by layer. In the FFF process, this happens by precisely extruding 3D printing materials in discrete tool paths that trace the outside of a layer and its celled infill.
The specifics of the format of the printer—its construction, what it’s made of, and the quality of its components— factor in to the quality and scale of the parts it can produce.