How to get a refund for a unauthorised subscription by Apple?

There is an Apple article with information about requesting refunds. Click on the following link: Request a refund for apps or content that you bought from Apple - Apple Support

Also, cancel the subscription if you haven't already: If you want to cancel a subscription from Apple - Apple Support

All you can do is try for a refund. It is stated by Apple that all transactions in the App Store are final, but in some cases Apple may provide a compassionate refund. It is not guaranteed.

A subscription from the App Store requires you to double click the side button on the device followed by verification with FaceID/Passcode, so it is not something that can be done accidentally. If you started a free trial, then you will need to cancel the trial at least 24 hours before the trial period ends to avoid charges for the subscription. The free trial will automatically start the subscription if you do not take action to cancel it. That is simply how all free trials work. As has been the case for many on this site, forgetting to cancel the free trial has not been a valid reason to grant a refund.