The answer is the save game, I’m afraid. But you have to get your around how saving actually works, because each thing that you want saved, has to know about that.
Anything that should have consistent status across different gameplay sessions, needs to save and load itself. If something gets destroyed, then it will know next time the game loads, it can just remove itself, and so on.
Doors that have been unlocked, can unlock themselves, etc.
If you think about it, there’s no way Epic, or anyone else, can implement a blanket save system, because they don’t know what kind of application you’re going to write using the engine, and what needs to be saved.