It used to always advertise being a media player so any time the app was active the volume buttons default changed the media volume. They just broke it intentionally.
Can't watch ads if you screen is off!
When I first subscribed to Google Play Music, in the UK, it included "YouTube Music Key", which had the same feature to allow YouTube videos to play in the background, and it worked just fine in the UK.
Then they rebranded YouTube Music Key to YouTube Red, and since YouTube Red isn't available in the UK for legal reasons unrelated to backgrounding YouTube videos, they disabled the backgrounding feature in the UK, even though UK subscribers are still paying for a YouTube Red subscription (that cannot be used in the UK).
[1]: https://play.google.com/store/apps/details?id=catchpower.gog...
With the new-ish priority mode and "do not disturb" modes, I change my ringer volume about as often as I change my "alarm" volume (which is almost never).
I only ever change my media volume, and thanks to apps like that one it can become the default.
On my phone it's under Settings -> Buttons -> Volume buttons -> Control ringtone volume.
On the Pixel it has a singular context aware volume control that doesn't work very well. I've loaded YouTube, pressed volume up/down, and had my ringer volume change more times than I can count.
Even on iOS which has a similar context aware volume control the experience is superior since you can also change the volume from the dock and disable the setting titled "[Ringer] Change with buttons" so it doesn't do anything until media is in focus.
I rarely need to change my ringer volume, I just need a quickly accessible way of going into "Do Not Disturb"/Silence mode for the cinema and meetings. Samsung even offers that from the power menu which is super convenient.