I feel like this could work using a tiny generator attached to the drive's motorized wheel, but that's probably too complex to be cost-effective for something like this unfortunately.
I can see the use if you're for example driving an older car with mostly original kit and don't want an anachronistic stereo in it. So you pair up your fake cassette to your modern phone and can still play Spotify or w/e with the original kit.
You'd be surprised, I've seen cars from as late as 08 that still had cassette. Though that's probably heavily dependent on manufacturer, model, region, and sub model type. But my point still stands, hell id wouldn't be surprised if there was a car or two manufactured in 2012 that still came stock with a cassette deck.
How does that work from the fake cassette to the player? Does the fake cassette record what's streaming to it to a loop of tape and let the player pick up the audio?
So a cassette tape works by using electromagnetism.
Ferric Oxide (AKA, literally rust powder) has a property that if exposed to a magnetic field, it will create a weak version of that magnetic field within itself
So the record head of a tape machine is an electromagnet that changes its field based on the actual audio signal, translating audio frequencies directly to magnetic directions and strengths, while the read head is a passive electromagnetic coil that picks up that weak magnetic field on the rust-coated plastic tape while a small motor runs the tape past it and emits it as a soundwave.
The tape adapter skips 90% of these steps —
— It just has an electromagnetic coil of its own, positioned so it lines up with the play head, and when you feed it an audio signal, that audio signal gets directly translated to a magnetic field just by running it through the coil. The tape deck picks it up and doesn't even realise there is no tape running through