It seems to me you've had your answer but don't want to believe it.
No, they don't use one single app. Here are some of the different apps they use:
- Fiio Music, developed by Fiio:
https://play.google.com/store/apps/details?id=com.fiio.music&hl=en_AU;
- Eddict Player, developed by Shanling:
https://play.google.com/store/apps/details?id=com.shanling.eddictplayer&hl=en_AU
- HiBy Music, developed by HiBy:
https://play.google.com/store/apps/details?id=com.hiby.music&hl=en_AU
They all look very similar because most music players on Android look similar, and most DAPs today are Android-based. If you want to assume they are the same exact app just because they look similar, then with the same logic you can assume Apple and Youtube are using the same app as these DAP makers, and 90% of all "music apps" you can find in google play store are just 1 app.
To add some extra data, here is the change log for FiiO Music from 2020 to 2024. This proves that at least this DAP maker has been developing its own app, for its players and the generic Android system, for over 4 years.