Are encrypted Wi-Fi passwords stored in
/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist? If yes, which string?
The best place to look for stored passwords for networks on macos in various user and system keychains. As an administrator, you can open Keychain Assistant and then look at the keychains (including ones synced with iCloud if you enable that in the preferences) for specific SSID and WiFi credentials that are saved.
These keychains are stored on the filesystem in encrypted form so you can use the
security command-line tool to read them if you don’t want to use the graphical version of the keychain tool.