How do I turn OFF "Hide my Email"

See the section titled "How to deactivate or delete an email address with Hide My Email" in the following support document: link-> Learn more about Hide My Email and iCloud+

Once deactivated, you would need to log in to the website you used it for, and change the email address to your actual email address.

Some websites will require you update your password too. As the Sign in with Apple feature, does not use one. But it will depend on the specific website in that case.