#3dprinting is rather unique in that every step in the process can be done with free and open source hard- and software. This ranges from design software that produces the 3D model, the slicer for the G-code, the firmware and even the bootloader of the 3D printer. On top of that all hardware parts can be 3D printed or bought.

I must add that not all manufacturers of 3D printers are adhering to the rules. The Prusa printers of Joseph Prusa belong probably to the most #FLOSS ones while Creality is currently doing an effort to be (partly) FLOSS. Others are probably in violation with the GPL license by using the Marlin firmware for their printers but not releasing their source code.

Some people argue that the design software is the achilles heel of this FLOSS workflow since Blender, OpenSCAD, FreeCAD and Solvespace all have their limitations compared to something like Fusion360. I'd say that for most if not all hobbyist the FLOSS programs are more than capable and no proprietary software is needed.