One of the most irritating things about YouTube is that you have to stay in the app if you want to keep the audio and video playing. So unless you have a Premium or Music Premium subscription, there's no option of leaving something on in the background while you do other things. Surprisingly that might be about to change, sort of, with users reporting that the app has started supporting Android Oreo's picture-in-picture mode - even for free users.
PiP support was already available for YouTube, but only for people who paid money for a Red (now Premium) subscription, but now people all over the US have started reporting that YouTube has been shrinking down into a mini video in the corner of their screen. That does make it slightly easier to keep a YouTube video running while you're doing something else on your device. That would make life much easier if you're listening to things like podcasts and music, though it's not quite the same as full background play that will carry on even if you lock the phone.
The downside is that YouTube has given creators the option to turn this feature off on their content, and so far a lot of musicians seem to have done exactly that.
While this test seems to be primarily based in the US, there are reports online of it popping up in other countries. Android Police mentions version 13.16.55, though considering my phone is running 13.23.59 that version might seem a little bit out of date. In either case neither of us could get PiP to work on our devices, but it shouldn't be long before the feature properly rolls out to everyone. [Android Police]