I believe it is called the backdrop.
Scenery is the effects that a person gives to a plaY for example costumes, background, lighting, sounds, and props.
scenery falls under the control of the set designer
Scenery is used to set the scene, mood and tone of a play. For example, a scene set in a garden may have a scenery involving trees and plants.
the person who changes the scenery between scenes are called...
I believe it is called simply a scenery "Drop" or "Fly"
The set is the scenery: the depiction of the physical location of the play.