The instructions from Apple for restoring from a backup say that all contents need to be erased first:  [Restore your device from an iCloud backup][1]

>If you’ve already set up your device, you will need to erase all of its content before following these steps to restore from your backup.

Does this burn the phone down to the ground?  I'm not interested in losing the current iOS 16.5 on it.   What does this "erase all contents" actually do?

  [1]: https://support.apple.com/en-my/HT204184#:~:text=Restore%20your%20device%20from%20an%20iCloud%20backup&text=Follow%20the%20onscreen%20setup%20steps,pick%20the%20most%20relevant%20one.