When I record in Audacity (vocal, guitar, mandolin, acoustic bass guitar), I always end up with the sound coming out of one channel. I have to go to audio track and then switch to mono so the balance is more in middle. When I record two windows open but one remainsempty (I delete it). And yet when I import a musical track both windows show the music!
You didn’t tell us what your interface/device is, but I bet it’s something like a Scarlett 2i2. It’s a stereo device, but it’s really two channel forced. It’s not a mixer. Whatever you plug into the left-hand connector is recorded as left and you’re pretty much stuck that way.
Some devices have the ability to appear as mono through software switches, but that’s not normal. Consult your instructions.
If you import a stereo sound file, it will appear as both tracks, left and right. That’s normal, too.
When you record with a full-on mixer…
…you have PAN controls which allow you to make the single microphone appear as Left, Right, or anywhere in the middle.
There are also Interface/Devices which naturally appear mono or stereo like the Behringer UM2 (also attached). It’s single microphone, no option and it can arrive as left part of stereo or a full-on mono track with settings in Audacity > Preferences > Devices > Recording (Attached).
Try swapping the settings on your interface and see what happens. Typically, the two-channel devices don’t switch.