The problem though is that you have to do this for every single site you access. So if you have 100 log ins and are switching PC or phone, you'll have to do this same dance 100 times in the next period. And of course, if you're switching because you lost your one device that was registered this way...
"Communicate over bluetooth" doesn't mean anything. What app or BT device would they be using? How would a PC communicate with a YubiKey over bluetooth?
I have no idea where you got this strange concept from, but registering multiple passkeys from multiple devices on the same account on a site requires no communication between the devices - it only requires a trusted device to approve the request.
No idea how Yubikey works, never used it.
Because surely such devices never get stolen, or dropped from a cliff.
FWIW I don't think that this makes passwords redundant in general, but with passkeys, password becomes a last-ditch safety valve to regain access to the account. Meaning that it can be generated, very long, and stored in a way that is optimized for safety and security over ease of access (like, say, an encrypted text file on multiple USB sticks stored in different physical locations).