Passkeys are typically synced to cloud storage.
https://developers.google.com/identity/passkeys/supported-en...
IMO if you're reading hacker news, you're fully capable of setting one up and leaving it in a safe locale for recovery.
What apps support this?
>Note: To use passkeys, iOS 16, iPadOS 16, macOS 13, or tvOS 16 (or later) is required. iCloud Keychain and two-factor authentication must also be turned on.
https://support.apple.com/guide/iphone/use-passkeys-to-sign-...