Have you considered reading the applicable developer documentation?
As with anything: try, fail, fix.
But given that these two desktop files already contains what they do - these are not “old apps”.
One has explicitly opted out, and the other seemingly tried and decided not to. So for the particular case you should consider asking the respective authors. I suspect they have their reasons - one of which may simply be available time.
Loading and playing mp3 files, and also whole directories full of mp3 files at once, works fine.
But Playlists don’t load music files. I can choose a m3u file from SD-card, tap ‘choose’ (Auswählen) (top/right button), then it jumps back to ‘Empty playlist’ on the screen. What’s missing?
Additional info: There are aliases to SD-card in ~/Music, ~/Playlists and ~/Videos on the phone, and path to Playlist folder on SD card is selectable and m3u files are visible. The alias in ~/ is not visible in Deadbeef’s file selecting screen if Sailjail is enabled.
edit: also I copied a m3u playlist into the internal memory’s Playlists folder for testing. It also didn’t work.