Audacity (and most programs) can’t split apart a mixed performance into individual voices, instruments and sounds.
The Level Speech add-on can try to figure out the voices are and adjust them. Sometimes that’s enough.
Scroll down.
https://forum.audacityteam.org/t/improved-leveling-for-speech/33258/1
This is why productions try to keep each person on its own track or recording.
Audacity can open up the sound tracks of many video formats. If it fails, you may need to install the FFMpeg software. Scroll down.
https://www.audacityteam.org/download/windows/
Audacity can take a video apart, but not put it back together. Export a corrected audio track from audacity and replace it with your video editor. Audacity won’t save a sound file. You have to export one. Start with WAV (Microsoft) 16-bit and try exporting MP3 if that fails.
Koz