Custom assets, such as 3D models, sound effects, textures, etc. are loaded from Unity Asset Bundles. Creating Asset Bundles requires the Unity 3D editor, which can be downloaded from http://www.unity3d.com.

Information on building asset bundles can be found on this page: https://docs.unity3d.com/Manual/BuildingAssetBundles.html

Assets from assetbundles are loaded using the global function LoadAssetFromBundle.

A .unitypackage containing a few sample assets, as well as some other helpful templates can be found in the Modding Files directory.