I think the OS is needed to sign the challenge, but the private key can be taken wherever. And passkeys are multiplatform. Not Apple-only, by a long shot. And

I think the OS is needed to sign the challenge, but the private key can be taken wherever. And passkeys are multiplatform. Not Apple-only, by a long shot. And I suspect that most geeks, hereabouts, could set up their own signing system.