I received a threatening email on my iPhone

Yes, millions of people have received the same bogus email. Delete the email and move on with your day. Do not interact with the message or the sender. No further steps are required.

Everyone receives junk emails. Just delete them and move on with your day. Your phone has not been hacked. They are just trying to scare you.

Some examples of those emails:

• May start out with the "Hey Pervert" line claiming to have photos of you or websites you visited that they will send to your contacts.
• Spoofing the sending address so you think that they sent the email from your own account.
• Include a Google Street View of your neighborhood claiming to be watching you
• Many demand bitcoin to be sent to them.

that is just

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Phishing - Wikipedia

if you really feel worried then try reading

If you think your Apple Account has been compromised - Apple Support

Your iPhone has not been hacked, you received a scam email in the hope that you would fall for the scam and send money to the scammer.

everyone can send you a pay claiming just that

it's what is called phshing