Why does audacity (3.0), while recording automatically make a stereo file.
When recording in multitrack mode (4 monotracks), audacitiy is automatically making each track into a stereo track.
How to maintain mono tracks??
Really 
I don’t see that here (on Linux). I get 4 mono tracks.
I’ve just checked with Audacity 3.0.0 on macOS (Big Sur) and I get 4 mono tracks there as well.
If you are looking to convert a stereo track into two mono tracks once they are created, Use the Split Stereo to Mono option in the Audio Track Dropdown Menu to split the two channels of a stereo pair into two separately editable mono tracks.