I tried to give a bit of an overview of this in the 'hardware design' section of the documentation [0]. It has some external links to further documentation and tutorials in it too. But the learning curve is very steep.

I'd maybe suggest looking for an electrical engineering intro book or course at a level you feel comfortable with, just to get an idea of the basics.

For the ErgoDox keyboard, the schematic is actually available on their own repo [1], but it's going to look quite intimidating initially. But I found a pretty great looking article explaining the electrical design of the ErgoDox that you might find useful [2]. The ErgoDox actually looks very similar electrically to the threeboard, it uses the same MCU (atmega32u4), but of course has the extra complexity of communicating with the other half of the keyboard over the 3.5mm connector.

[0]: https://github.com/taylorconor/threeboard/blob/master/docume... [1]: https://github.com/zsa/docs/blob/master/ErgoDox%20EZ%20Schem... [2]: https://kandepet.com/dissecting-the-ergodox-the-ergonomic-pr...