Object in Blender Object mode
Object in Windows 3d
Object in Unity
It looks like a triangle winding problem. The order of the vertices in a triangle, when combined with their visual orientation, can be used to determine whether the triangle is being seen from the "front" or the "back" side. In Unity, by default triangles seen from the "back" are not rendered. This is called backface culling.
To fix this you have two options:
Cull Off
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