How can I set my iPhone 13 to automatically backup daily?

See the following from the Apple support article How to back up your iPhone or iPad with iCloud - Apple Support

Automatically back up with iCloud Backup

1. Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.
2. Connect your device to a power source.
3. Connect your device to a Wi-Fi network.
4. Make sure that your device's screen is locked.
5. If you get an alert says that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage.

Note that once it is set up, it should automatically backup while it charges if it has a wi-fi connection and the screen is locked. You can lock it with the right side button or just let it lock itself based on the time selected in Settings > Display & Brightness > Auto-Lock.