When you download a trial app, you don't get a notice that the trial is over and you will be charged. If you don't properly cancel the trial subscription the day before the trial period is over, by virtue of downloading the app, you have authorized payment for the paid subscription.
Properly cancel the paid subscription -
If you want to cancel a subscription from Apple - Apple Support
Ask Apple Support for a refund -
1-800-MY-APPLE
M1 Mac mini, macOS 12.5; Watch, series 5 GPS/cellular, watchOS 8.7; iPad Pro 2nd Gen, iPadOS 15.6; iPhone 12 Pro, iOS 15.6; TV 4th Gen, tvOS 15.6; HomePods, audioOS 15.5.1 and Airport Express w/ Airplay 2