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 persistent 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 persistent 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