I just wondering how modems work.

I know the process of "modulation" and "demodulation".

So I wanto know is "demodulation" are simple revese of "modulation" in real world.

The PC can got an IP address when connect as type 1

What will happen when I connect them like type 2?

type 1 :

[PC] ================= [router]

type 2 :

[PC] === [m] ------- [m] === [router]

[m] : modem(exactly same)

=== : Ethernet cable

--- : DSL cable (phone line, maybe optical fiber, or something else?)

ISP has a large number of model, Is them same thing as the one in my home(but with diffrent size/speed/price...etc)? Or it's completely different thing?