GPIO ​

https://www.raspberrypi.com/documentation/computers/os.html#gpio-and-the-40-pin-header

https://www.raspberrypi.org/documentation/usage/gpio/ GPIO - Raspberry Pi Documentation

https://howchoo.com/pi/add-raspberry-pi-4-cooling-fan
How to Add a Raspberry Pi 4 Cooling Fan - Howchoo

https://duckduckgo.com/?t=ffab&q=raspberry+pi+gpio+layout&ia=web
raspberry pi gpio layout at DuckDuckGo
https://duckduckgo.com/?t=ffab&q=pi+4+fan+pin&ia=web
pi 4 fan pin at DuckDuckGo