http://valinet6l6tq6d5yohaa6gdsf2ho4qcqcxyj2nahp5kd4z7nsa6ycdqd.onion/2021/01/20/Automatically-backup-the-iPhone-to-the-Raspberry-Pi.html
If you previously enabled encryption in iTunes, you can skip this probably. To be able to decrypt the data, you have to set a password (if you used this previously, you have set this as well). This password is different from the iCloud password and is stored only locally on your iPhone.