What works with offline Siri in iOS 15?
You don’t need to enable offline Siri in iOS 15; it’s on by default and works with commands like:
Opening apps
Controlling system settings like dark/light mode, volume, Accessibility features, Airplane mode, Low Power mode, etc.
Setting and changing timers and alarms
Playing the next or previous song in the Music app
https://9to5mac.com/2021/09/10/how-offline-siri-works-iphone-in-ios-15/