Google searches have only yielded 5th grade level examples (“the modem talks between your ISP and your home network!”) or articles I would need a degree to understand. Can anyone provide an explanation that’s somewhere in between the two? I understand the fundamentals of how the Internet works, and how LAN works regarding a router and individual devices, but I’m curious to know more about the link between those.
Take a look at old issues of Byte Magazine where they will explain how to make a cassette tape backup system for your 1975 computer. Basically, you need to turn ones and zeros into tones that can be recorded on cassette tape, and you need to do it in a way that strings of 111111’s and 00000’s get the right count of '1’s and '0’s. There are a bunch of different ways of mapping tone changes to '1’s and '0’s, (frequency shift keying, phase shift keying, etc) and it’s pretty amazing all the ways to pack bits into tones (how many tones do you want to distinguish - 2? 4? more?).
You could also look up how the old 300 baud (essentially bits per second) modems encoded bits over telephone lines that only provided 3Khz of bandwidth, and how more sophisticated encodings allowed 1200 baud, 2400 baud, and ultimately 56K baud.