I don't use the home assistant integration, but my understanding is you have to configure WiFi and an MQTT server on a node at your house with the meshtastic app (I probably wouldn't use a T-Watch for that, something with a longer external antenna would be better), and point it to your home assistant's MQTT server. Then there's some more configuration to make home assistant send messages and what to do when it receives messages.
Most meshtastic compatible devices do have a USB port that presents as a serial interface though, so there's probably an unofficial integration that makes use of that too.