With X Rebirth, player-created content used to change the way the game works is called "extensions". In earlier Egosoft games, player-created content is called scripts or mods.
Please note that the use of extensions or save game edits can break games either of themselves, after future game updates, or via conflict with other extensions. Consequently, Egosoft are not generally able to support modded/save-game editted games (see here for further details).
NZ-Wanderer manages a fantastic, up-to-date list of many extensions, and their status, here (on Egosoft's X Rebirth - Scripts and Modding
Unless noted otherwise, extensions are generally 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 connivance for the user in that the Steam will manage installation and updating.
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.
Save Game Editing Tricks are described here.
Overview
Content Tools