In this case, the app is running in a sandbox with a user-space that simulates the iOS user-space [0].

> Your apps use the same frameworks and infrastructure that Mac Catalyst apps use to run, but without the need to recompile for the Mac platform.

> Although you can run your iOS apps unmodified on a Mac with Apple silicon, Mac Catalyst lets you build your app specifically for macOS and customize your app’s behavior on that platform.

Mac Catalyst was a multi-year effort by Apple. Doing the same to run macOS apps on iOS would probably be even harder due to how complicated macOS is compared to iOS.

[0] https://developer.apple.com/documentation/apple-silicon/runn...