as the title says basically.
AFAIk it’s neither a PS/2 or USB keyboard, it is directly connected to the mainboard, maybe using ps/2 protocol, I don’t know. But for sure, it doesn’t require usb 
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It is a nice subject to cover as it involves an advanced type of hypothetical physical attack which might be relevant to some Qubes OS users. If you have ever disassembled a laptop, you might have recognized the ribbon cable attaching the physical keyboard (and sometime touchpad, pointing stick, backlight, …) to the motherboard.
There are active projects of documenting electrical/physical specification of those ribbon cables, developing hardware & software solutions to decode the communication protocol between the laptop motherboard and the keyboard. Here is an example.
It is possible for an evil maid attack of installing an interceptor device between the keyboard and the mainboard to communicate the keystrokes over 4G or other wireless protocols.
p.s. But I have not answered your original question. What happens after the ribbon cable on the mainboard? It is sometimes USB but most of the times it is PS2. Look into /dev/input/by-path. Your keyboard controller is most likely listed there. You could search for it and find more information on it.
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