To start a free trial, it requires you to double click the side button with the trial period showing on the screen, along with the subscription start date/price and a message that says to avoid charges you must cancel at least 24 hours before the trial period ends. That is what you agreed to when starting the free trial.
All subscription will automatically renew until cancelled by you. In the App Store, all transactions are final, but at times Apple may provide a compassionate refund. It should never be expected though.
No, Apple does not monitor your activity on your device so they have no idea if you have used any app or not. That really does not matter anyway as you could have a Netflix subscription and will still need to pay whether you watch any shows for a specific month or not. To avoid future charges, the subscription would need to be cancelled.