Any in depth basic explanations about World Composition?

Hi , i am looking to find out more about World comp , an explanation of exactly how it works , Like if you have a large map divided into sections(squares) , how does the main persistant level relate to all the sub levels. and if you have a directional light or a sun sphere like a time of day and weather system , does that go in the main persistant level? And then still works for all the sub levels? And if you are putting mesh’s and stuff in , do you have to be working in that particular sub level that relates to that part of the map where you are placing them? Also , if you start with an empty map and sculpt it , then later on when you add , say , mesh’s and foliage to the maps , will those also be automatically streamed like the empty landscape is? What about things that cross the border of each adjacent section (level) of the map ? I have seen some different documentation on World Comp , but nothing that explains all the basics or in depth on it? Thanks for any help

Like also for example , if i import an 8129 x 8129 heightmap in and component size is 254 x 254 , leaving 1024 squares. Am i supposed to work with that many squares? Seems like a lot.

And so if i load levels , and save everything , why are all my levels unloaded again when i restart the editor?

And if i am trying to put an auto landscape layer on the landscape , it seems i can only make current one tile at a time and do it, but this would take me weeks.