Full card numbers are not stored on the device or on Apple Pay servers. Instead, a unique device account number is created, encrypted, and then stored in the Secure Element. This unique device account number is encrypted so that Apple cannot access it.

The device account number in the Secure Element is never stored on Apple Pay servers or backed up to iCloud, and is isolated from iOS, iPadOS, and watchOS devices, as well as Mac computers with Touch ID.

More information about Apple Pay privacy can be found here.