If you do not cancel the subscription before 24 hours from the last date of trial, Apple can attempt to start the subscription by deducting the amount as per the selected plan.
According to Apple, when you sign up for a free or discounted trial subscription and you don't want to renew it, you need to cancel it at least 24 hours before the trial ends. The chargeable subscription begins automatically on completion of the free trial duration, so the app works without any interruption.
The trial is an initial offer and is offered through AppStore utilizing your Apple Account. Anyone can avail the trial just one time. The trial and subscription are managed through Apple.