Did you cancel the free trial properly?
Managing auto-renewing subscriptions -
If you want to cancel a subscription from Apple - Apple Support
You might need to scroll down to find the Cancel Subscription button.
Scroll further down the link for instructions for non-Apple devices.
Things about subscriptions;
⁃ Deleting an app does not cancel the subscription.
⁃ A trial subscription must be canceled more than 24 hours before the renewal date or it may renew.
⁃ Many subscriptions are set to renew automatically until you cancel them.
⁃ If you see multiple subscription offers, as long as you don’t select one, the subscription shouldn’t renew.
⁃ If you don’t see Cancel Subscription, the subscription is canceled and won't renew.
⁃ An expiration date means the current subscription has been canceled but is available to use until that date.
⁃ Use Resubscribe/Renew if you change your mind about canceling the subscription.
If all else fails, check out Apple Subscription Support for articles and contact options;
https://getsupport.apple.com/?PGF=PGF63006&category_id=SC0060&symptom_id=23208
I you didn't actually cancel the trial and it converted to a paid subscription after the 7 days, the charge is legit and Apple rarely gives refunds.
Ask for a refund -
Request a refund for apps or content that you bought from Apple - Apple Support
M1 Mac mini, macOS 15.1; Watch, series 10 GPS-only, watchOS 11.1; iPhone 16, iOS 18.1; TV 4th Gen, tvOS 18.1; HomePods, audioOS 18.1 and Airport Express w/ Airplay 2