There are two ways for Home Assistant to consume the information produced by zigbee2mqtt.
Automatic discovery
There’s an option in zigbee2mqtt to have it publish the sensor data in a way that Home Assistant will detect and automatically add (to the entity registry) as new sensors.
Manual
You define each sensor manually in the configuration file. This is more work but gives you complete control over each sensor’s definition.
If you’re not currently using either method, the sensors will not appear in Home Assistant.
Someone else using zigbee2mqtt recently required fine-grained control over the sensor’s definition so I suggested they use the manual-configuration approach: