Unexpected charges for free apps on iPhone

An app may be designate as Free to download, but most will have in-app purchases or free trials that will become a subscription. If you sign up for a free trial, you will see the trial period on the screen where you are required to double click the side button. Along with the trial period, you will see the subscription start date, price and the message that says to avoid the subscription, you must cancel a day before the start date of the subscription.

You are not charged simply for downloading a free app and any charge will have to be approved on the Authorization Screen described above where you have to double click the side button, followed by verification with Touch ID or Passcode.

If you currently have a subscription from a free trial that you did not cancel, you can cancel those here:

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

Perhaps the app is free but requires a subscription.

Check here.

View your purchase history for the App Store and other Apple media services - Apple Support