YouTube has an another app called “YouTube Music” designed specifically for playing music. It mostly just gives you the option to play only the audio of a given video (saving on bandwidth, etc). Only works if you have YouTube premium/red or whatever it’s called.

Might be called something different in other regions (I’m in the US).

They might mean YouTube red? Which is the premium version of YouTube where videos keep playing in the background on your phone and no ads so it’s mainly used for music. It’s just the YouTube app itself however.

Pardon I think it’s called YouTube premium here in the US now. It was YouTube Red.