How do you input the voltage signal to an oscilloscope?
My guess is that he has a program that sends that key sequence continuously while he adjusts his test setup to capture it correctly.
That's not just a keyboard, it is a computer.  He has a program running that is sending those characters through the network port.  This involves about a dozen layers of hardware and software, such as the PHY (physical layer) and MAC (Media Access Controller), both of which reside on the NIC (Network Interface Controller).  Software takes the code for a keypress coming from the keyboard interface, converts it to ASCII, sends it to more software such as the &quot;IP stack&quot;.  The entire system seems bizarrely complex at first, but each layer of the onion was added for a reason.
Here is a description of a protocol stack, to give you a taste of what's going on in there.
https://forum.allaboutcircuits.com/