Since iOS 13 you've had the option to use Sign in with Apple to register with any apps, websites or services that offer it. Basically it is like signing up for something using your Google or Facebook credentials, but it doesn't track and profile you like they do. If you have been using Sign in with Apple you can view and manage which of your apps and services are using it in your iPhone or iPad Settings. Here's how:
Sign in with Apple
Sign in with Apple offers a secure way to use your Apple ID across a variety of apps, websites, and services. When using Sign in with Apple, every account is automatically protected by two-factor authentication, and signing in via iPhone or iPad only requires Face ID or Touch ID. Apple even offers a way to hide your real email address when creating new accounts, with built-in forwarding to protect privacy.
Sign in with Apple uses an existing Apple ID to quickly and securely sign in to websites, apps, and services. To access an account using Sign in with Apple, just tap the button and confirm your identity with Face ID or Touch ID.
Sign in with Apple can be used anywhere you can access an account, app, or service. This means Sign in with Apple works on web browsers, Android and Windows apps, and all Apple devices.
Sign in with Apple can be used anywhere, as long as the app or website offers Sign in with Apple as a sign in method.
Sign in with Apple automatically requires two-factor authentication (2FA) to protect your Apple ID. This means every account or app logged into using Sign in with Apple is also protected by 2FA.