Configuration.yaml and MQTT

Hi there. I’m new to Hass.io, just installed on my Raspberry 3.
The installation went smoothly and now I’m trying to get the MQTT interface working.
I installed Mosquitto, tested it offline (two ssh sessions, transmitting and receiving) and it works.
Then I integrated MQTT in hass, and it works, both sending and receiving.
But…

  1. Where the hell does Hass write this configuration? I can’t find any “mqtt” entry in the “configuration.yaml” file. And, as far as I can find, there is only one “configuration.yaml” file.

  2. How can I change the MQTT configuration?

Thanks for your patience.