I opted to not renew my subscription to YouTube Premium several times this month but Apple/iTunes renewed my subscription anyway sometime over the last 12 hours and I desperately need a refund

Hi jonmcmurray,

Commisserations!

Seems cancellations should take effect after the current billing period ?

This may or may not work for you, as I do not know what country you are making purchases from.

How to request a refund

1. Go to reportaproblem.apple.com. 
2. Sign in with your Apple ID and password.
3. If you see the "Report" or "Report a Problem" button next to the item that you want to request a refund for, click it.
4. Follow the instructions on the page to choose the reason why you want a refund and submit your request.

Or

You might want to click on the "Get Support" link at the right on the top of this page to contact Apple.

Also

Apple support have articles online that may be helpful:

Check the status of a refund for apps or content that you bought from Apple

(Includes: When will I get the money back?).

https://support.apple.com/en-au/HT210904

Or

Request a refund for apps or content that you bought from Apple:

https://support.apple.com/en-au/HT204084

and

How long refunds from the App Store or other Apple services take:

https://support.apple.com/en-au/HT210904

Hope something here works for you :-)