I’m not sure about the STEP file format, but the IGES format exported by FreeCAD outputs planes (144 - Trimmed Parametric Surface) bounded by (142 - Curve on a Parametric Surface) lines (110 - Line) for the case of the brick. There are twelve of these sequences … one for each triangle in the model. I suspect that any SketchUp STL export of lines and faces will translate into the same IGES entity types with no problems. I’m not familiar with STEP files, but I’ll look into its format when I get a chance.
Side note: From about 1982-1990 I wrote a number of IGES importers in FORTRAN for GE Calma systems. Most of the issues revolved around inconsistencies between various exporters, notably defining the normal vector to an arc … most exporters used the right-hand rule and some, for whatever reason, used the left-hand rule. When I’d see a radiused part that looked like this:
I had to reverse the sense of the vectors during the import. It’s somewhat depressing to see that folks are still using Hollerith ASCII format ;(