Help Creating A Universal Remote hub

I'm trying to create a universal remote to replace a harmony hub and I have no idea how to start.

I think you need bit more precise. You have

lirc

in Linux that can receive and transmit IR codes that can be used for this purpose. Otherwise would I recommend you to take a look at Arduino and MQTT as there is for example several "MQTT-IR bridge" project based on Arduino/ESP8266/ESP32.

thanks I will look up on that.

Well the MQTT-IR bridge is quite versatile, where you can send any "IR Code" in the MQTT package