Hi Melstorm.
I did some additional research on this. HLS is an abbreviation for HTTP Live Streaming. It breaks video files up into much smaller chunks and adds a second file that instructs the player on how to put them together. The result is much better performance for streaming video on a multitude of devices and varying network conditions. Bottom line is that this format is made to be played in a web browser or streaming video player which is why you can't see or open them in QuickTime.
The TV app uses a library that is on the internal drive and there is currently not a way to have that library access files from two different locations. Your best option may be to use the capability of the app to play content from multiple libraries. You would create a library (not just the downloaded files) on the external drive and launch the TV app to address the external library when you want to access your backed-up files.
Use multiple libraries in the Apple TV app on Mac - Apple Support
As far as the current inability of the library to access files from multiple locations, if you would like to leave feedback for Apple about that, you can do so here:
Feedback - macOS - Apple
Using the drop-down menu for Feedback Type, choose the option for Feature Request. In the drop-down menu for Feedback Area, choose Applications/Utilities
Cheers.
