First the layers - layers work so that what is more close to the screen (to reader) is on higher layer than what is in distance.
Here explained more in detail:
Regarding the zooms - the best is to try to experiment with zoom - there is really no rule on how to use them it is more about the directing style. Zooms help to make the story visually more attractive. So the best way is probably just to experiment with them a bit to find your own style.
While having a dialogue aspecially if it is a longer one zooms can help to keep reader in. What you can for example
start the scene with reset zoom and then make slow zoom on both the characeters.
When one of them says something important zoom only on him. Or then zoom only on the other to show reaction (like if he is surprised atc)
You can use zoom to increase feeling…for example girl is running away because somebody is hunting her…you can zoom just on the running legs which will increase the uncertainity who is it and why is he after her.
Or you can make shaking efect with zooms… or even use easing function to adjust your zooms
There are for sure many other posibilities how to use zooms.