StepUp is a plugin / workbench for FreeCAD and acts as an interface between KiCad and FreeCAD.
I have not really used it myself, but as far as I know a lot of the 3D components in KiCad are made in FreeCAD and then exported.
I just did (one of my first) experiments with StepUp. You can directly load a KiCad PCB in FreeCAD with it:
The PCB tracks have to be loaded in a separate step (5 icons to the right).
A simple search should point you in the direction of some documentation:
StepUp can apparently also be used to design (complex) footprints for KiCad in FreeCAD, but I never tried that myself. FreeCAD is not a very easy program to learn, and I’m only familiar with a small sub section of FreeCAD’s capabilities.