Yeah, I was hoping for more detail as well. I'm guessing they did something similar to what I did to make my Pi control computers.[0]

The Raspberry Pi 4 can emulate a USB keyboard and mouse, and there are inexpensive adapters that allow it to capture display output. You can also hook it up to a relay to cycle power for an external device.

[0] https://mtlynch.io/tinypilot/