Ben Eater on keyboards
I have recently been enjoying some of Ben Eater's videos on low-level hardware and implementing physical interfaces.
In particular, the series on how a USB keyboard works, how USB device discovery works and how n-key rollover works are measured and methodical – literally measured, as in hooking up the leads to an oscilloscope and going from electrical to digital.
My nose is sometimes deep into network/IPC protocol design these days, and so I appreciate the similarities and differences between software-only and protocols that have to bridge two pieces of hardware. Sockets can be frustrating, but you don't have to worry about wires turning into antennas, differential signaling, clock desyncing and bit insertion.
See also, or perhaps start with, the PS/2 keyboard interface video, which gets both practical and visually deadpan. (After watching it, I feel like I understand better what you'd use the Raspberry Pi Pico's PIO layer for.)