i bet that they work the same way that the cd to cassette adapters worked except, instead of a stereo aux jack with continuous audio signal, it uses an digital to analog converter and the rest of it works the same way.

here’s video from on of my favorite youtubers who does a really great job at explaining how consumer electronics work and he explains how the cd to casette & bluetooth adapters work: https://www.youtube.com/watch?v=dH4n8fUjtLQ

Same way they did when it was a wire to a 3.5mm plug, just a Bluetooth module now.