system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do services such as Spotify and Overdrive make sure that you don't permanently download their material?

Certain services, such as Spotify and Overdrive, allow you to download music or books. But you can't keep these files: Spotify, for example, would never let you keep them unless you continue your premium subscription—or else you could just download everything, unsubscribe, and then have all the music in the world. So how do they make sure that users—despite having downloaded the content already—not simply keep it for themselves?
gpt
The files are not in a standard format. You couldn't just take the files and play them using another media player app.

The Spotify player knows how to decode the files into a normal audio stream and then sends it to your sound card. If the process used to encode the Spotify tracks was known, then it could be reverse engineered and a conversion utility could be made. That of course would break the  Terms and Conditions contract you agree to as a user and could also be illegal based on the laws of where you live.