I just got the link between MQTT and ZIGBEE2MQT
For information, I have:
Enable the Raspberry Pi serial port to communicate with an external module
sudo raspi-config
Interfacing options
Serial
…
Additional steps to enable the serial port for Raspberry Pi 3, 4 and later.
Link to the manipulation: Enable the Raspberry Pi TTL serial port and check that it works. (raspberry-pi.fr)