There are a number of zigbee2mqtt tutorials on the forum. Here’s one: Zigbee2mqtt revisited: No more ugly transformations
Creating and configuring Generic MQTT Things in the UI is covered in Adding Things - Advanced | openHAB.
At a high level, you create the Thing. Then you create a Channel that represents one of values reported by the device. Configure the Channel to subscribe to the topic (e.g. zigbee2mqtt/smoke_detector_01). Configure a transformation to convert what ever message gets published there to something that can be used by an Item.
Often, as appears to be the case here, you will have one Topic that multiple Channels subscribes to with a different transformation for each Channel to convert that big JSON package to individual Item states.