Why would I receive an email stating I have purchased "Netflix Premium"?

You may be the subject of a scam.

Did you provide any potentially damaging information to anyone? If yes, you'll need to follow up, for example, with a credit card company, Apple, your bank, etc.

If you gave you Apple ID password to a scammer, then

Change your Apple ID password https://support.apple.com/en-us/HT201355

Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

Learn about Phishing, Scams, Apple Practices,etc.

 See If you see apple.com/bill, itunes.com/bill, or an unfamiliar charge on your statement https://support.apple.com/en-us/HT201382

See Identify legitimate emails from the App Store or iTunes Store https://support.apple.com/en-us/HT201679

See How to avoid scams when using Apple Pay to send and receive money https://support.apple.com/en-us/HT208226

For scam related information from Apple including reporting scams to Apple see Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams https://support.apple.com/en-us/HT204759

Identify scams related to purported apple notificationsper Eric Root.

“Apple will always address you by your name or the name they have on file for you, not Dear Customer, Dear Client or by using your e-mail address. The e-mail will be from @apple.com or @iTunes.com. E-mail addresses can be spoofed. You can go to Mail/View/Message/Show all Headers to see more. Apple emails won't have poor grammar/misspellings. Apple e-mails will never contain an attachment. Apple will never request personal information by email such as Social Security numbers, your Mother’s maiden name or full credit card numbers. …

“The only exception to the above I have noticed is if you order something from the Apple Store (apple.com), your receipt will be addressed to Dear Apple Customer. That is a receipt for a purchase you initiated.”

Learn about/Increase security

See If you think your Apple ID has been compromised https://support.apple.com/en-us/HT204145

Increase the Security of your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

You purchased Netflix Premium or you are being scammed.

About Email and Other Scams

Apple always addresses you by your name or the name they have on file for you.  The Email will be from apple.com or itunes.com. Email addresses can be spoofed. You can go to Mail > View > Message > Show all Headers to see more. Apple Emails will never contain an attachment. Apple will never request personal information such as Social Security numbers.

An exception to the above is when you order something from the Apple Store. The receipt will be addressed to Dear Apple Customer.

Avoid Scams

1. Identify and report phishing emails and other suspicious messages
2. Annual Reminder- Watch out for email Phishing Scams
3. Identifying fraudulent "phishing" email
4. Beware of Browser Pop-Up Tech Support, Phishing Scams
5. Phishing & Other Suspicious Emails
6. Avoid phishing emails, fake 'virus' alerts, phony support calls, & other scams
7. Phony "tech support" - "ransomware" popups and ... | Communities

Report phishing email to abuse@apple.com.