You can view the sleep score on your iPhone or on your Apple Watch. On the watch (Series 6 or later), go to the Sleep app.

Mirroring an Apple Watch to your iPhone might at first sound redundant. What can you do on an iPhone that you can’t on an Apple Watch? The feature, available in Settings > Accessibility > Apple Watch ...

Make sure Apple Watch and old iPhone are updated and synced to iCloud > back up old phone > set up new phone. When asked to use Apple Watch, select Continue > keep devices close > follow on-screen ...