Unintended subscription charge after free trial on iPhone

You agreed to the charge when you signed up for the free trial. On the page where you must double click the side button to confirm the free trial, you saw:

• The trial end date
• The subscription start date
• The subscription price
• A message stating that you must cancel at least 24 hours before the subscription start date to avoid charges.

I would recommend setting a reminder when starting a free trial, before the subscription date in case you do forget. Once you see the subscription charge it is too late. In most cases, all transactions in the App Store are final since it does take that deliberate action on your part to agree to the terms. At times Apple may provide a compassionate refund, but it should never be expected.

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

You must cancel at least 24 hours before the renewal date.

If you want to cancel a subscription from Apple - Apple Support