Where are light and shadowmaps stored in open world?

I want to make an open world but also underworlds and other setups with a lot of light sources. I tried using dynamic light only which kinda worked in the open world but once dungeons come into play, it is basically impossible to have more than a handful of fully dynamic lights without dropping the framerate. Shadow is the main culprit but I also don’t want a setting with bad shadows.

So, I am going back to backed light. My question is, where is the big chunk of light and shadow stored? In the level data for the persistent level or in the respective sub level of the open world that holds the terrain?

Thanks