Hi @passio
On Apple devices, new passkeys are synchronized using iCloud synchronization. You can read about it here. Unfortunately, Apple has removed support for passkeys that are not synchronized.
However, this now means that you should be able to seamlessly login into you II on all your Apple devices that use the same iCloud account.
If you are uncomfortable with synchronized passkeys, your best option at the moment is to buy a YubiKey and use that to sign into II (and remove the Apple devices).