I’m stealing this from another comment:

The main advantage comes with phishing resistance. Standard MFA (time based codes) is not phishing resistant. Users can be social engineered into giving up a password and MFA token. Other MFA types, such as pop up notifications, are susceptible to MFA fatigue. Similar to YubiKeys, Passkeys implement a phishing resistant MFA by storing an encryption key, along with requiring a biometric. The benefit here is that these are far easier for the average user, and the user does not need to carry a physical device. Sure, fingerprints could possibly be grabbed with physical presence, but there is far less risk that a users fingerprint is stolen, than a user being social engineered over the phone into giving creds. For most organizations and users, this is far more secure.

You can still keep password + 2FA on GitHub and Google Suite (probably anything else that’s currently implementing them), it’s just a convenience/anti-phishing feature right now.

The passkey is synced between devices if it’s kept in a password manager, I haven’t looked at the mechanism that Apple uses to sync it/use it if you store it in the system keychain. I guess you could also have multiple passkeys configured for a few devices.