alt Hacker News> You can't copy them out on at least the iOS, Android, and (to my knowledge) Windows default implementations.
Pardon? The official support docs disagree with you [0][1][2]. They absolutely leave the device.
Other passkey managers let them leave the device in a way that you control, but even the default ones copy them off the system they were created on.
[0] <https://support.google.com/accounts/answer/6197437?hl=en&co=...>
[1] <https://support.apple.com/guide/iphone/passwords-devices-iph...>
[2] Examine the "Can I use passkeys across multiple devices?" Q and its A here: <https://support.microsoft.com/en-us/windows/passkeys-frequen...>
Yes, they're synchronized, but I wouldn't call that "copying them out", as that to me implies somehow getting access to the raw private key or root secret bytes.
Both Apple and Google have pretty elaborate ceremonies for adding a new device to an existing account in a way that synchronizes over passkeys.