It’s a good question. A lot of 2FA apps have manual backup/restore functionality. Some have cloud sync (e.g. iCloud sync so your new iPhone has the same app and codes, or 1Password/Bitwarden which has you log back into the app on the new phone with their service login.) These 2FA syncs can be a point of weakness so not everyone uses them.

The services themselves (rubygems etc.) also provide a short list of one-time account recovery codes. You’re supposed to essentially print them and put them in a safe. I wonder how many people both keep those codes and keep them somewhere secure…