Doing a large scene, also rgb curve

newbie here. I was wondering. I see a lot of tutorials of people doing a plane or a dropback for there scene set up. if I was doing a large scene should I just make a large square box over the scene and use it as an emission with a sky texture?

the other question is compisiting in render I guess. I ask this but knowbody answered. I was wondering how do they do this affect in these short animation films made in Blender? is it called render passes? is it just mist with light rays? [ATTACH=CONFIG]280296[/ATTACH][ATTACH=CONFIG]280297[/ATTACH][ATTACH=CONFIG]280298[/ATTACH]