tutorial video reference

unity docs reference

Exporting prefab into bundle

1. Go to Window > AssetBundle Browser
2. Drag your prefab into the left side of the Configure window and create a bundle with it.
   If there's already a bundle there, you can also click on the bundle and then drag the prefab into the right side to add the prefab to the bundle.
   It's up to you how you want to organize your bundles, but all you really need to keep in mind is that you don't need to drag in every sub-prefab or material, because they will get added automatically (shown as greyed out)
3. Go to Build tab, and change the Output Path to the desired location (this will either be a folder in your code project (the one with the .csproj file in it) or a folder in your mod folder (the one in OuterWildsModManager/OWML/Mods)
4. Click Build! That's it! It's easy.

Importing prefab into New Horizons

See the examples here