This is based on the open standards WebAuthn and FIDO2, where the credentials (“passkeys”) are synced via iCloud Keychain. Currently you need remember to register at least 2 security keys, in case one is lost/misplaced. The syncing of passkeys in iCloud solves this backup problem.

https://fidoalliance.org/apple-google-and-microsoft-commit-t...

How will this work on Linux?

I wrote a FIDO implementation that protects the signing key using the system's TPM specifically for linux: https://github.com/psanford/tpm-fido

There is no reason why you couldn't implement a similar syncing strategy in a tool like this if you wanted to.