There are various approach to solid modeling.
Some systems supports a hybrid solution.
In the CSG approach the solid is described as a combination of elementary solids like cubes, spheres or extrusions. The actual faces are not computed the result is obtained by a shaded image. CSG are very practical for rapidly adapting display. Checking properties like if a point is inside is based on checking the same property on the elementary solids. Construction of mechanical parts including filets or shells is very complex.