AR Plane Manager allows the app to detect horizontal planes in the device's camera's view. AR Raycast Manager allows the app to identify which horizontal plane a raycast from the direction of the camera would strike. Our script fires a ray each time the device screen is pressed (clicked), and the location of the first horizontal plane hit by a raycast is the location where a new clone of the prefab is created.
By tagging AR Camera as Main Camera, we can easily get a reference to the camera using Camera.main.
By placing the instance object of our script class as a component of the AR Session Origin GameObject, we can get a reference to the ARRayCastManager sibling script component by using GetComponent<class>() in the Awake() method.
The only public variable we have to set for the instance object of our FurnitureManager script class in the AR Session Origin GameObject is the prefab of whatever piece of furniture we want to be creating clones...