Sinks are outputs, basically “speakers”. Run pactl list sources to list inputs (“microphones”).
It’s not very complicated in my opinion once you understand how parts fit together. The easiest way to change from where an application records is to open pavucontrol, go to the “Recording” tab, find the application, then change its source to whichever audio source (so external microphone input in your case) you desire. This is the preferred and easiest way.