OK, I think I figured this out.
The current version of "Files by Google" does not have the necessary functions included to for this head unit. (Actually, it may not have the proper files for any Android 10 device with this problem.)
THIS SHOULD BE CHECKED AND NOTED/CORRECTED IN THE Poweramp FAQ.
Installing the proper version of "documentsui.apk" for Android 10 solves the problem on my Atoto Android head unit.
details of my question on XDA here:
https://forum.xda-developers.com/t/atoto-s8-general-discussion.4114493/page-33#post-86956181
And the answer here:
https://forum.xda-developers.com/t/atoto-s8-general-discussion.4114493/page-33#post-86956471