A user types at a terminal (a physical teletype). This terminal is connected through a pair of wires to a UART (Universal Asynchronous Receiver and Transmitter) on the computer.
Once I had the opportunity to sit at such a machine.
A real typewriter with real paper. And no display! Only paper.
I type my "command line" and it appears on my paper. Ok, where else...
The computer responds by letting it's response appear - what do you think? - on my paper, too. Not by magic, but by my machine typing it.
I am typing to my paper. Then the computer is typing to my paper. And so on.
Hardly ever have I had such a creepy feeling while doing computer work 😵💫