Astronomy | is it an app or wallpaper |

Are you referring to the new iPhone iOS 16 wallpapers, including the Astronomy theme (Earth, Earth Detail, Moon, Moon Detail, Solar System)? It is part if the iOS system software, and not an app as such (although it has app-like properties, like Earth Detail zooming in on your location, and appearance customization (e.g. clock font and overlay widgets)).

Change the wallpaper on your iPhone - Apple Support

Wallpaper is a photo, either directly or constructed.

Wallpaper does not require access to any other information.

Apps can and variously do request access to information.

Who is “they” here?

What app or image or files has been installed, and from where was it acquired?

Is this an iPhone, iPad, Mac, or some other platform?