Yes, it's implementation specific, in this case your phone, or your browser is the passkey "device". And as long as it's protected by some form of authentication it's OK (though I would recommend a hardware token over phones/browsers). If it doesn't then you shouldn't be using that "passkey". Yes, there is no way for the website you are authenticating with to know whether your passkey is safe or not, choosing a secure passkey implementation is (unfortunately) the user's job. But it's the same with more traditional 2FAs, e.g. you can store your TOTP secret securely or insecurely, and the website will have no way to know.