Generally, the idea is that the private keys are stored on a TPM chip so they cannot be exported for security reasons, similar to a YubiKey or a crypto wallet. I know Apple and Google are looking into trying to allow users to somehow export these keys elsewhere, but I'm uncertain how easy that will be or what tradeoffs there will be. I'm skeptical that Apple and Google will try to make it too easy since they benefit from user lock-in, but I would be happy to be proven wrong.