If you have multiple microphones / sound cards, in the pavucontrol app (or in pwvucontrol), you can set which one is the default ("fallback device") for output and which one is the default ("fallback device") for input, by clicking the corresponding checkbox-like togglebutton in on the right in the "Output Devices" or "Input Devices" tab.

However, on startup (and live when you change the default and the app is not currently recording), Reco does not preselect the system-set fallback/default device, it selects whatever is first "at the top" of the list of devices, as you can see here:

It would be very practical and convenient (especially in rushed recording scenarios) if Reco could honor the system's "fallback" default device properties, at least on app startup, and ideally also when the app is already running and the user changes the default and the app was not already recording (though in the "already recording" case, maybe it can prompt the user and offer to switch to the new default stream?)