The 'Sign in with Apple' button lets you create accounts with third party sites and services using your Apple ID, without needing to create a new password or provide additional information. This is similar to how you can use your Google or Facebook login to access certain apps and websites.
The issue is in Apple's implementation of the button. The company is forcing any app developer that uses another third party sign-in option to also implement 'Sign in with Apple'. If they don't, their app will be rejected from the App Store.
Apple says its “Sign in with Apple” button, which gives users the ability to hide their personal details from the app, is “pro privacy.” But investigators are examining how Apple uses the sign-in button and other App Store rules to make it more difficult for users to switch to a rival device maker, according to the two people, who have met regularly with the Justice Department over the past 18 months. The government’s interest in the Apple sign-in button, which the company introduced in 2019, hasn’t been previously reported.
More details in the full report linked below. You can download the iClarified app or follow iClarified on Twitter, Facebook, YouTube, and RSS for updates on the investigation.