1. The both work the same as far as the final output. In high poly scenes, you break up the scenes into scenes, and if you don’t have the system resource you then break it up into separate files. So you may have a background file/scene, the animation file/scene, and a foreground/environment effect file/scene. In the animation file/scene, you would use simple objects with the hold material, because that uses less resource than using the holdout collection. Although it is very small difference if you have a compatible GPU; however, if you use only CPU rendering it makes a noticeable difference when rendering an animation. Remember a fraction of a second on a single image adds up when you’re doing 24, 25, 30, or more frames per second.
2. Alpha over is the normal way.
3. It can be used to achieve this affect, but it’s not necessary for occlusion culling since blender already uses this method instead of Frustum culling. However Blender does use Frustum culling for the lights in eevee. This may have been changed in eevee next(Latest version)
