3D files, usually .STL and .OBJ, are resized or scaled up/down before they are sent to printer which typically uses .GCODE (or "go codes) to translate the 3D file to print the item. Kind of like putting a piece of paper on a copy machine and use reduce or enlarge.
Slicer (configured for a specific printer) software is often used to convert the 3D file to a GCODE file.
One can find "viewers" to look at the 3D file or even free 3D file editor such as Meshmixer to edit files. (Microsoft has "3D Viewer" in 10 but not 11. Free download on Microsoft. )
(Note that the GCODE file mentioned above is not a 3D file, but a directions file like used in NC machining.)
I hope this simple explanation shows the usual process of how 3D files are often printed and avoids misinformation on who to seek out to do this.
Lew
Florida ⛱️, USA
Also, you can check out the flow process I created about Shapeways.
https://lewsmodelboats.org/3D-models/ShapewaysProcess.png