MechKey.Org - All about keyboard

PCB Designer Guide by ai03
Keyboard PCB Guide by ruiqimao
A modern handwiring guide
Hand wiring a let's split (pro micro)
How to integrate a trackpoint in a mechanical keyboard (TMK, Teensy)
lets-split-guide - This guide covers building a Let's Split v2.
the 1-Key-Keyboard project - One key keyboard using Attiny85.
4-key-keyboard - Four key keyboard using Attiny85.
zmk design guide by eblaster - While it targets zmk keyboards the lessons apply wider than zmk