Why do you want to import an obj file into Unity for terrain? If you want to create terrain externally it’s best to import a RAW heightmap file versus a OBJ file. Main reason being performance. If you import a highly detail terrain mesh into Unity you’ll probably have hundreds of thousands of vertices.
The terrain system Unity utilizes is more optimized and can work with your custom terrain if you use a heightmap instead. You can also edit that heightmap in Unity as well. This was covered near the end of this course.
