Steam-based X Rebirth
Steam Workshop mods are added to the extensions folder in the game's installation directory ( e.g. C:\SteamLibrary\SteamApps\common\X Rebirth\extensions ).
Extensions can be subscribed to via the Steam Workshop. This offers convenience for the user in that the Steam will manage installation and updating.
gog-based X Rebirth
XRWSunpack tool for using Steam Workshop mods with gog-based X Rebith is here.
All X Rebirth users
Extensions can also be found on the Nexus website. This alternative route can take the player a little longer to use but it can offer the modder increased flexibility.
Unless stated otherwise, manually downloaded mods will also be added to the extensions directory in your X Rebirth installation.
Save Game Editing Tricks are described here.