basic principles: luarc files
At startup, darktable will automatically run the Lua scripts $DARKTABLE/share/darktable/luarc and $HOME/.config/darktable/luarc (where $DARKTABLE represents the darktable installation directory and $HOME represents your home directory).
This is the only time darktable will run Lua scripts by itself. These scripts can register callbacks to perform actions on various darktable events. This callback mechanism is the primary method of triggering lua actions.