How to find out when and where backup recovery password was set for iPhone backups?
The password used for local iPhone backups is set when enabling encrypted backups. When checking the box to use encrypted backups, a dialog asks you to enter a new password. This is independent to your Apple ID or other passwords.
Your Mac stores the password in the login Keychain and uses it for future backups. To get the password from the Keychain, open Keychain Access.app, search for ‘iPhone backup’ or ‘iOS backup’ and open the item, then choose ‘Show password’ and enter your Mac account password to view the backup password.