as Meike already pointed out, the letter you received might be a fake letter. You can easily check this: Did they describe the circumstances, e.g the exact time you were supposed to have downloaded the movie or your IP-address?
If you are not sure what to do, you can send a copy to the consumer care service and ask them for clarification. They offer online office hours for such cases, f.e. here: https://www.meine-verbraucherzentrale.de/DE-BY/emailberatung
I hope my answers were useful for you and I'll be very pleased if you contact me again with further questions. I'm a member of the mbeon-staff and I‘ll be glad to guide over a longer period of time.
mbeon is an online advisory service for migrants via the mbeon-app, which you can download on your smartphone for free. You'll find me in the mbeon-app under my name München, Frau Müller and get confidential one-to-one advice in a safe virtual room where nobody can read messages. For further information please visit www.mbeon.de.
Download for Android: https://play.google.com/store/apps/details?id=de.zone35.mbeon
Download for iOS: https://apps.apple.com/de/app/mbeon-messengerberatung/id1408795643