If you modify anything the level “references”, then you’ll need to resave the level. You may even have to run a build on the level in some instances.
The level updates itself as you modify BP actors etc that are placed in the level. For example if you change a mesh in a placed BP it will automatically update in the level.
change a sm component to from dynamic to static, or modify shadow settings … level will need to be saved and a light build run.