[Open World] Get object collection before start

Hello my friends, please can you tell me how to…

I’m trying to develop game such as Fallout, Skyrim, etc. which on huge are so many objects. If we using level composition and dynamic loading objects - we don’t know about objects in non loaded parts.

So, how to get objects palced at levels before starting, activating or load level? Or how to do something like that? or right way?

I see only one way, thats: connect to editor and handle save level event, produce custom hierarchy and save near *.umap. After loading build - load custom hierarchy at object model.
There is normally? or there is more effective way to do that?

Thanks!