You can create meshes with different levels of detail in an external 3D application for use with Unity’s LOD system. If you name these meshes correctly, Unity automatically creates and configures a GameObject with LOD group component for them when it imports the Model.
To import a Model with LOD level into Unity, you must do the following:
In your external 3D application, follow the application’s process to create as many LOD meshes as you need.
Export your Model as an FBX file. Alternatively, if you are using Maya, export the mesh group directly into Unity; to do this, go to File > Send to Unity > Selection.
Import the FBX into Unity. Unity recognizes the grouped Meshes and naming convention, and automatically creates an LOD Group component with the appropriate settings.
To learn more about importing models, see Importing models.
To learn more about creating and exporting LOD Meshes in Maya, follow the Unity tutorial Creating LODs in Maya.