unknown app signed into my Apple ID and it won’t let me delete
the app is also nowhere to be found.
iPhone 15 Pro, iOS 18
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