How does two Factor authentication work with one phone if it breaks

If you truly don’t have any other telephone number you trust to add to your AppleID, then you would have to either use account recovery (How to use account recovery when you can’t reset your Apple ID password - Apple Support) or, setup beforehand an account recovery key and use that (How to generate a recovery key - Apple Support).

With the account recovery 28 digit key, it is imperative you write it down or store it on paper or electronically somewhere you can always gain access to it. The advantage of setting the key is you can always recover your AppleID, but it means if you lose or forget the key, there is no way to recover it.

BTW, for a second trusted telephone number, it can be any number including VOIP service numbers like Google Voice numbers, Ooma, Skype - any of those, or any valid telephone number can receive a 2FA code as backups.

A) if you have another AppleID device (iPad, iPod touch or Mac) as a trusted device under your AppleID, you would get a code there whenever setting up an new iPhone or new Apple device. 2FA codes, by default, are sent as iCloud encrypted push notifications to all your apple trusted devices.

B) if no other Apple device you would receive a code as an SMS text or automated voice message sent to your backup trusted phone number you added to your AppleID. If your only Apple device is an iPhone, it is especially vital to set up a second, backup trusted telephone number listed under your AppleID.

Two-factor authentication for Apple ID - Apple Support