I have a few things connected to openHAB via zigbee2mqtt and Mosquitto. @chrode has linked the post that got me started with it all. I wrote the following tutorials which might be useful:
Bulb - IKEA Tradfri dimming bulb
Bulbs - 3x IKEA Tradfri GU10 dimming bulbs
Switch - Xiaomi multi-press button
Switch - IKEA Tradfri dimming switch