Reddit reviews Audio AUX Car Stereo Cassette Tape Player Adapter 3.5mm Headphone Jack Converter Plug for MP3 CD DVD MD Smart Phone

Essentially cassette tapes contain magnetically stored information that a tape head reads as the tape is passed over it. The cassette tape adapters which plug into CD players don't have a spinning tape, but it instead has a device that passes the audio information to the tape head reader in the car stereo as magnetic information. You may say there is a magnetic generator in the tape that tricks the tape head that there is a tape being read.

For those who may had difficulty understanding what you meant, it took me a few reads at least, this is the kind of tape he's referring to: https://www.amazon.com/Stereo-Cassette-Adapter-Headphone-Converter/dp/B07G8WWF9C