[sowbug]

Blue Pills and Black Pills are popular with the DIY mechanical keyboard crowd as the brains of a keyboard circuit. The Arduino Pro Micro is also popular. I imagine that nowadays people also use the Raspberry Pi RP2040.

The common feature among these boards is that they have relatively full-featured USB client support in hardware. This is different from other hobbyist embedded boards, such as most Arduinos, for example, which have only a USB virtual serial interface via a discrete chip for the purpose of uploading and debugging firmware. So if you can make the board look like a USB keyboard, and it has enough I/O to handle a matrix of keyboard switches, then with the right firmware like QMK, you've got yourself the building blocks of a custom keyboard.

My info is stale as of the start of the pandemic, when I gave up my mechanical-keyboard hobby and pivoted to DSLR-based videoconferencing.