There is no mqtt.cfg file any longer. You need to install the MQTT Binding. Then create an MQTT Broker Thing with the connection information to the MQTT broker. Then you create Generic MQTT Thing and configure Channels on those Things to publish and subscribe to the right topics for these devices.
This is covered in Adding Things - Advanced | openHAB
I don’t know anything about Tuya but I assume that the information you gathered will help tell you what MQTT topics to publish and subscribe to. Step-by-Step guide for adding Tuya-bulbs, Wi-Fi smart LED (Smart Life app) to OH2 using tuya-mqtt.js by AgentK and Tuya devices to Openhab via MQTT - a working solution (without flashing)! should help with that.