Nope, it cannot be done without you knowing as it requires you to double click the side button with any charges showing clearly on the screen before they are authorized when using an in-app purchase that is managed by Apple.
As for what an app offers with in-app purchases, that is up to them on what features they want to make available and how much it would cost. Some apps will give you access to features through one time purchases for individual content, or subscriptions that will be renewed automatically depending on the length of time for the subscription.
You can view your Purchase History and view/cancel any subscriptions you have that are managed by Apple here:
If you do not see the subscription/purchase in those links, then that means that you have made a subscription with the developer of the app directly on their website and it is not being managed by Apple. Cancelling this type of subscription can only be done by using that website where it was started. More information can be provided on what you find out using those 2 previous links.
The first step is to cancel any subscription or future charges. After that is done you can write a review for the app in the App Store to alert other users about your experience and also report the app to Apple as a scam if you wish. You report the app to Apple by:
- Sign in to reportaproblem.apple.com.
- Tap or click "I'd like to," then choose "Report as a scam"
I searched for the app in the App Store, but there are many to choose from, so not sure which specific app you are referring to or who the Developer is.