Receiving an email at your backup email address? Getting a text message? It’s whatever you have decided to set up in your account, the same as with a password.
If the worry is your phone getting lost then use a password manager that syncs to the cloud and log in with the same passkey using another device. I just got a new iPhone and logged into a bunch of things on-device using synced passkeys. It worked just fine.
Every conversation about this seems to involve someone asserting that improving upon passwords in any way at all is unacceptable unless the solution is perfect in every possible way, with the goalposts constantly moving.