This is the folder all your mods will go into. The first step is finding your game profile folder (where your save files are located). For that you can use this guide:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1777493789
(see guide for Linux/macos/win store)

Once there you might see a Mods folder if you've played around in the mod editor, if not then create it and name it Mods.

Now that you have a Mods folder; you need a mod to install. Once you've downloaded and extracted the mod to a folder, you simply move the mod folder to the Mods folder.

To make sure a mod is installed check for Mods\mod name\metadata.lua

Last, but not least is to enable the mod in the in-game mod manager.