So how does a PS/2 keyboard interface work?

There's a good video on how PS/2 works here: [VIDEO] It's pretty straight-forward. There are also pre-made kits for digitizing typewriters if that interests you.

Piggy-backing this comment to add some context for the OP video: This "USB device discovery" video is actually the latest in a series of videos where Ben Eater is building a simple 8-bit computer on breadboards. He hooked up a PS/2 keyboard as an input device, and then got sucked into a tangent of investigating and explaining keyboard protocols. - He investigates the PS/2 keyboard protocol: [VIDEO] - Somebody on Patreon asked about USB keyboards having higher latency than PS/2, so he made a video about the basic USB protocol: [LINK] - He made the OP video about USB keyboards being limited to holding six keys at once.

think might be at end of this one < [VIDEO] > though there's a few he's done recently on USB spec

I'd imagine it would be very difficult, and would require a relatively powerful MCU. Ben Eater's made a few videos discussing how PS/2[0] and USB keyboards[1] work. USB's a vastly more complicated protocol compared to PS/2. [0] [VIDEO] [1] [LINK]