Passkey is some sort of specific unique key to a device allowing to use a pin on a device instead of the password. But which won't work on another device.
Now I don't know if that key can be stolen or not, or if it's really more secure or not, as people have really unsecure pins.
There is no implementation right now that enables you to own and manage your own passkey backups without Google it icloud.
Additionally, the attestation feature is one step away from banks and other sites mandating specific implementations, preventing people from using software tokens or OSS managers.
Passkeys is great, and I am eager to recommend it to everyone, but without those items addressed, it's a trap door, and one bitflip away from very strong lock in.
I just set my passkey up in Dashlane. It does not require Google or iCloud.
The same webauthn standard allows you to use a security key with PIN luck