You’ve mostly got it
As you correctly deduce “Audacity projects” are stored in two parts - there is the .AUP file that has the Audacity logo, and there is a “_data” folder. The AUP file is usually quite small and contains a list of instructions for Audacity - it does not contain any audio data. The audio data is stored separately, usually in the “_data” folder.
To create a normal audio file from an Audacity project you must “Export” the audio. “File menu > Export Audio”
I regularly search high and low, everywhere except for where the thing I’m looking for actually is
The full article is in the manual: http://manual.audacityteam.org/o/man/audacity_projects.html