Some of the best moddable game examples:
Skyrim, Witcher 3, Cities Skylines.
They all have an offline/out of game somewhat intuitive scripting system that is based on either a very customized language and needs a special editor (bethesda) or text based (witcher/skylines).
They also usually offer the same commands within the game itself via console commands. Particularly skyrim, where you could literally script a whole new level by spawning and positioning things around via copy/paste.
It really all depends on the amount of time you wish to spend vs what it is that you wish the players to customize.
For skyrim you can format and pack new 3d objects and animations because they re- created a basic game editor essentially.
The Unreal examples are actually in the forums below. Don’t get too excited, the games and the mod systems for those kinda suck. In fact I’d disregard them altogether and suggest implementing your own scripting system instead.
You could actually just wrap a c++ conversion of your (whatever)Basic language (darkBasic, visualBasic, probably another 10 at least) and effectively allow the language to be used for scripting (in a limited fashion unless you really want to re-wrap all the functions).
Then naturally you would need a system that loads the files. Not hard to do, but all of the games cited above have brilliant examples.
1 file determines the loading hierarchy of the rest. Some have prerequisites so that the file can be organized appropriately right inside the game launcher, some (witcher) do not and depend on the user doing the legwork.
Some others simply load all the files within a specific folder.