These pin-socket headers are for making your microcontroller easily replaceable without having to desolder. They're perfect for builds such as the ScottoNum, ScottoErgo, or Scotto34 (both handwired and PCB Edition).
Sold in one pair of 1x40 pins, just enough for an Arduino Pro Micro or Raspberry Pi Pico. Simply use a blade to cut the length you need, solder the gold side to the microcontroller, and connect the socket side to the keyboard.
If using these on a handwired build, you may have to sand them before gluing to get the perfect fit.