You open up 'general', you go to 'subscriptions', and select that. Any subscription on the iPhone can be canceled right there. As this is done through Apple's payment processing, Apple/iPhone knows about every subscription and can cancel it easily.

However, if the subscription is processed through a third party, then you may have more difficulty with canceling it... but it wasn't bought through the app on the phone.

Additionally, by default, renewal receipts are sent for each subscription and are visible under the purchase history.

If Epic wins and you've got subscriptions set up using payment processors other than Apple, this will make it harder to manage these subscriptions.