i don't think you really understand how it works. most implementations are secondary implementation alongside login+password.

how to use cross-device? either use some password manager that supports it (apple/google/1password/keepass/etc support it), or use device that you have on hand most of the time - phone. when the passkey pops up - point your camera and scan the qrcode - you are done. otherwise use dedicated device like yubikey or similar.

really not sure what is hard about that to understand. i'm using android and chrome, so i can use the password manager in chrome, or my phone to scann the qrcode.

my country is using similar authorization for government "profile" (mobywatel - poland) that has similar to passkey implementation. you download the app on your phone, login via login+password (or other), download the certificates, and from now on you can point your camera on qrcodes to login to government websites; it requires pin/code or biometric confirmation on the phone - same as passkeys.