unknown app signed into my Apple ID and it won’t let me delete

It says it could also be a website. Have you never heard of that one? You have apparently used it before.

What is Sign in with Apple? Click here --> What is Sign in with Apple? - Apple Support = What is Sign in with Apple? - Apple Support

"When you see a Sign in with Apple button on a participating app or website, it means you can set up an account using your Apple ID. No need to use a social media account, fill out forms, or choose another new password."

Click on this link to learn how to use Sign in with Apple --> How to use Sign in with Apple - Apple Support = How to use Sign in with Apple - Apple Support

Read about managing and removing here Manage your apps with Sign in with Apple - Apple Support

You don't specify as to exactly what the problem is.

If an app or feature on your device is not working, not appearing, or is restricted (grayed out), a frequent reason is it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution.

- Read this support article If your Apple ID or other options are greyed out in Settings —> If your Apple Account is grayed out in Settings - Apple Support

Tap Settings > Screen Time > Content & Privacy > scroll to Allow Changes and turn on.

- If the device is managed (Settings > General > VPN & Device Management), contact the IT department that is doing the managing.