How do I back up my iPhone automatically?

Turn on iCloud Backup, plug the iPhone into power, keep it on Wi-Fi, and leave the screen locked. Apple says that is the setup for automatic backups, so once it is switched on the phone can save a fresh copy without you having to remember it every time.

iCloud Backup stores things like device settings, Home Screen layout, and app organization that do not already sync to iCloud. It does not replace syncing, though. If you already use iCloud Photos or Messages in iCloud, those items may be stored through sync instead of the backup itself. The other thing to watch is storage space: if iCloud is full, the backup will not finish until you clear space or buy more.

That separation matters because a synced photo or message and a backup are not the same thing. Sync keeps the current version available across devices; the backup gives you a restore point if the phone is lost, replaced, or wiped.