Privacy policy URL for app
According to https://developer.apple.com/app-store/review/guidelines/#privacy you only need a privacy policy if your app has account sign in capabilities.


  17.5 Apps that include account registration or access a user’s existing account must include a privacy policy or they will be rejected


If your app just links to other apps and they handle the sign in, you should be clear without needing a privacy policy. You could also rely on iOS to handle the account log in - same release from the requirement is met in that case.