Mixing relates to timing and crowding. You don't want the melody or main theme being dominated by other parts in the same sonic range. You can adjust various part levels, EQ, entry points, and duration, to make the important part stand out. And you don't want busy parts in different ranges getting un-needed attention. You want to support and embellish the main part. Sometimes the instrument or voice of the main part isn't going to work, and an alternative is needed.
The type of song you're making also matters. There are industry standards and audience expectations for various genres. I have a few dozen songs that I occasionally listen to, and each time, I appreciate the uncluttered performance, with each part well presented, and not in competition with other parts of the song. Rick Beato has some 'what makes this song great' videos, that get into details, a playlist:
https://www.youtube.com/playlist?list=P ... aiM7ryfXV_
Rhythm and tempo also matter. Sometimes some swing or syncopation can liven things up, and if a track drags, or seems rushed, it can be changed.
Audacity has an alternate 'play' button, with a tempo slider, that's handy for experiments.