ARK: Survival Evolved

I’ve never played this game nor modded it, but its modding scene is one of the most “well off” due to the mod kit that the developers provided. Their mod kit is a release of their entire game UE project on top of their custom engine version. It means that modders can play the game in-editor to test their mods.

They distribute this by providing a separate ~100GB download for users that own the game.

The mod kit is so special, that it has its own entire category on the Unreal Engine forums!