https://github.com/SmartlyDressedGames/U3-Docs
There are lots of aspects for modding, so reading this whole thing would be good.
You would need Unity version 2020.3.38f1 LTS as well, if you intend to mod new items that looks/sounds different from vanilla items.
After making the things and they are in a folder, open Unturned, click workshop and there you can see you can submit new workshop items yourself.
Just need to input the mod's name, the folder's location, a random image's location then you can hit the submit button, wait a bit and your new mod is out in the workshop!