What is the name of the app?
Is the app in your purchase history?
If you see the app in your Purchase History, then the download required double clicking the side button, followed by verification with Face ID or Passcode, so that is not something that can be done by accident and if it was not done by you, it was done by someone that know your Passcode with physical access to your iPhone.
If you used a free trial, then as always those will become a subscription if not cancelled at least 24 hours before the trial period ends. Those also require the same authorization/verification as any other purchase.
The most important part is determining how this would have happened as you said you do not recognize the app and would not have made a subscription, yet don't seem concerned at all that it would mean your phone or payment method has been compromised and needs immediate attention to prevent any further or more costly charges. Are we missing something here?