How do I create a single mp.3 file from a Music app playlist on Mac?

Not really with the Music app. You should use an audio editor for that.

(You could kind of hack it if you have a CD drive; put the songs into a playlist, burn to CD with no gaps, then rip back in Music using Join Tracks to keep them all together. But the transitions will sound bad.)

You are welcome. To make a nice continuous MP3 you need to normalize the volume of all the individual files, and adjust the transitions to that they are cross-faded and (preferably) beat-matched. I used to do a lot of that sort of thing with Audacity. It is far beyond what the Music app can do.