The author is using PowerAmp.

I am using AIMP. I was specifically looking for apps with downloadable APKs for sideloading. I can't use the Play Store because I don't have a google account. Looks like PowerAmp also has an APK.

I'd also very strongly recommend F-Droid. TBH I can't really speak to the quality of music apps on it - I just use VLC - but it doesn't require an account and it has lots of good apps.

I use Auxio from F-Droid. It's quite simple (which to me is a benefit) and my only real complaint is how similar shuffle on and off look on Android Auto. It handles non ASCII titles correctly where some others didn't (from what I remember due to weirdness with the Android media store) and it picks up new media/edited quickly and reliably which can be a sore spot with others. I don't know how Android's media store is apparently so incompetent that most music apps don't work correctly using it.

can also vouch for Auxio - happy user of 5 years

I use the F-Droid apps Transistor to listen to streaming radio and Antennapod for podcasts on an old phone:

https://f-droid.org/en/packages/org.y20k.transistor/

https://f-droid.org/en/packages/de.danoeh.antennapod/

Use Aurora Store to download apps from Play Store without an account (very trustworthy app IMO, has been around and working for years):

https://auroraoss.com/aurora-store

PowerAmp is paid, and Aurora Store can't do paid apps or in-app purchases without a Google Account. It's better to download the APK and buy a license key on the official website, which doesn't require Google Play Services if you do it that way.

Looks like they also allow you to buy with a credit card on their website, and they say Google Services aren't required if you buy that way. Very cool!