I received notification from a hacker.

Your iPhone can’t be hacked into. Apple has already protected it. If you got a message saying your phone had been hacked it was a scam to steal your identity by tricking you into clicking a link in the message and entering personal information. See→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Why would someone warn you if they were going to hack into your phone?

Think of it this way, if someone was breaking into your house, do you think they would call you and let you know that?