Installing the Unity Editor is required for exporting custom content into the game. Most 2021.3 LTS version should be compatible; Unturned currently uses 2021.3.29f1. View Download Links
Once Unity is installed a project can be created to house custom content. At this point it is recommended to import Unturned’s provided source packages:
Inside Unity open the Assets > Import Package > Custom Package… wizard.
Find the Unturned installation directory.
Navigate to the Extras/Sources directory.
Import the Project.unitypackage, and optionally the ExampleAssets.unitypackage.
This package contains the bare-bones required to export custom content:
This package contains vanilla content examples, and several useful prefabs:
CoreMasterBundle directory has at least one example of each piece of vanilla content.
Animations directory has all of the vanilla item animations for re-use.
Assets/Resources/Characters/Preview.prefab is helpful for previewing clothes.
Note that custom content should NOT be placed into the CoreMasterBundle, instead create a separate directory.