You may get a OTP when adding your card for verification, but not on every purchase. I assume you have reviewed your Purchase History here:
View your purchase history for the App Store and other Apple media services - Apple Support
If you signed up for a free trial and did not cancel it, then you will automatically be charged for a subscription. You can view and manage your subscriptions here:
View or cancel app subscriptions on iPhone - Apple Support (MT)
If you have given someone the passcode to your iPhone, you have authorized them to use it and that includes purchases, unless you proactively added restrictions to prevent that.
If you are a Family Organizer and use Purchase Sharing then the family members are authorized to make purchase to your payment method.
If you feel your account has been compromised, then you should immediately change your Password and Passcode to your device.
Change your Apple ID password - Apple Support
Set a passcode on iPhone - Apple Support (IN)
You can request a refund by following this link. Refunds are not guaranteed, but at times Apple may offer a compassionate refund.
Request a refund for apps or content that you bought from Apple - Apple Support
If the charges do not show in your Purchase History, then you will need to consider if your card details have been compromised and resolve that issue with your bank.