The recovery process is defined for both Apple and google, as is the device transfer process.
Here’s a google post explaining their recovery process: https://security.googleblog.com/2022/10/SecurityofPasskeysin...
Before that, some phones could already offer their embedded security chip as a U2F authenticator with the browser app. E.g. I setup the Titan chip in my Pixel 5a as another authenticator with a Google account. But, it only works with a browser on the phone itself. Since the phone browser can also access a Yubikey, I could use the Titan chip to enroll a new Yubikey or vice versa.
I do wish there was a standalone "security key" USB mode I could select alongside file transfer, picture transfer, tethering, or charge-only. Then, I could more reasonably use the phone's embedded security chip as a redundant U2F key for many different services I would normally only access from a PC.
The way to deal with losing an authenticator is you have multiple of them, including several backups, one in your home safe, one at your friend's place etc.