Distinguish infill from empty

[Chainsaw]

In the SketchUp model in the image, a section shows a volume I've painted gray above one in burgundy. The colours have nothing to do with the final 3D print—I'm just trying to show what part I want to be solid infill and what part the printer should leave empty. The gray volume is a typical one in the model that I intend to be a wall separating spaces like the burgundy space. What I don't understand is how the STL file that is eventually derive from my Sketchup file conveys to the printer which part of the model is to remain solid and which to leave vacant. Since Sketchup only erects volume boundaries with infinitesimally thin planes, what is to distinguish solid from space other than what seems to be irrelevant colour. Or do I need to do my model in a true solid modeller because Sketchup is not a solid modeller?

[Bikeracer2020]

Sketchup is well known for producing poor files for 3D printing, plenty of free to use modelling programs out there.
A Google search will bring up a good variety, I use Fusion 360.
Any software will have a learning curve, but Google has plenty of tutorials.