An STL is just a mesh, a collection of joined up points. It's like exporting a JPEG of a word document, you can print it but editing is a huge pain. For instance STLs don't support curves, so they're approximated with lots of short lines.

Usually CAD interchange is either in STEP or the program's proprietary format.

You would expect STEP files in an open source project. This allows you to change the motor interface to use locally available stepper motors, adapters, or other modifications. STL is an intermediary, "rendered" 3D STEP file, ready for conversion into final custom gcode assembled specifically for your machine and any accessories/user specific procedural steps/initiation procedure etc.

STEP is the sheet music, MP3 is the STL and the waveform is the gcode.

STL is to CAD as JPG is to SVG basically.

More like source code vs object code in a compiled computer program.

The STL is a mesh file that is used to generate g-code that a 3d printer or CNC machine understands.

The CAD file, whichever format its in, is going to have all the solid modelling information and be much more useful if you want to make changes.

Freeware vs. Open-Source. PNG vs. SVG.