You’ll get a better answer on something like r/embedded. I.e. seek out the people who work on them.

Here’s the less-then-degree answer: Think about a telephone dial tone or key press sound. That’s sending a digital command over audio. You’re sending a tone that the other side can pick up and turn back into a number. You can step up higher and higher in frequency to send more information. This was originally what they called broadband, when it got high enough in frequency to go outside the audible range. There’s other techniques besides just tones, like phase differences between two signals (think how headsets can mimic directional sound). You could use this to encode much more information than just frequency alone. As modems get “faster” we’re moving to more complicated algorithms to send data.