And how do you interface to it? Do you send what you caught to MQTT?

I am considering having something else as the RF receiver but would need to forward what it received to MQTT (I can write that part myself - it is really the interface to the dongle I would like to understand)

I use DVB-T USB dongle. You can use this project[1] to decode it.

[1]https://github.com/merbanan/rtl_433