Most materials in Unity will render single-sided. This is unlike the defaults of many modelling programs. The direction of model faces is determined by the winding order of the vertices that make up a triangle.
Research how to invert/flip normals in your modelling program. Additional key words to search for are face normals and backface culling.
Select the inverted faces and flip their normals.
Backface Culling is single-sided rendering, enable it in your modelling program to see how a model will appear in Unity.
If an object has partially flipped normals there may be a recalculate normals operation to unify the normal direction.
Unity, huh, how?