Passkeys are just using WebAuthn as the primary authentication method. WebAuthn works by users having a separate private key for each site, with the sites storing a copy of the public key, and sign-in works conceptually by the site generating a nonce, which must be signed by the private key, with the signature presented back to the site who can verify it against the public key.

The problem though is that WebAuthn is most convenient when using the platform authenticator (so you don't need to plug in a U2F style dongle). But conceptually the platform authenticator is tied to a single device. (Because the private keys are supposed to be stored within a TPM or secure element of some form). But that means if you lose/damage the device you are locked out.

Obviously one option is to have a lockout flow that asks security questions, or sends an email or something, but obviously those are not nearly as secure as private key stored on a device you need to physically possess. Plus generally you need to authenticate to the device too, with biometrics, or pin, or something, before WebAuthn flow can proceed, making these flows inherently two factor (something you have, plus either who you are for biometrics, or something you know for pins).

The original solution for this was to always register a roaming key (like the U2F style dongle), as a backup, and as a method to initially sign in on a new device (aftwe which you could enroll the platform authenticator).

But nobody does that, so instead most platforms are also storing a copy of these passkeys in their associated clouds, meaning that if you lose your device, you can get a new one of the same type, sign in it the Microsoft/Google/Apple account, and you can now sign into sites that use WebAuthn as sole authentication again.

But this only helps when moving between devices in the same ecosystem, what if you only have an account on your iPhone, and want to sign in on a Windows PC? The new idea is to allow this via scanning a QR code, and verifying that the two devices are physically near each other (verified via bluetooth low energy). (So this will work for laptops, but not all desktops have bluetooth). Obviously once you sign in like this once, you could then add the new devices platform authenticator as a sign in method, so you don't need to do this every time.

This cross-device QR sign-in approach is being standardized via FIDO, although as far as I can tell there is no published specification right now. Draft versions of FIDO specifications are not publicly available, with the exception of the WebAuthn spec, since that is developed jointly with W3C, under the W3C process. But most of this would be on the CTAP side of FIDO2, which is a pure FIDO developed specification, so the only way to see the current draft specification would be to become a FIDO member.