Your progress is either saved locally on your device, or in cloud. When you connect Angry Birds POP to your Facebook account or ‘Sign in with Apple’, your progress will get saved online, and you will be able to restore it in case you need to reinstall the game or change devices.
You cannot link the same progress to both Facebook and ‘Sign in with Apple’. ‘Sign in with Apple’ is available on devices with iOS 13 and above, and it does not include the social features that Facebook does (e.g. level specific leader boards and profile pictures).