Gyroid fill type
If you want something very resistant, set it to 25%.
3D printing is not 100% solid, but rather a waste of material, nor is it hollow.
nor hollow but any 3D print when it is being created, would be deformed.
I wanted to know what it was like inside, so I paused a print and removed the material.
The fill is the infill option (20% material and 80% air by default)