It doesn't have a native app - it's just a web page. If the browser supports it, background playback might work. Alternatively you can try zap.stream/videos which shows them as well (in a different player). 

 The brave browser will actually allow you play your videos in the background while your screen is off.  Just open youtube in brave and play the video, but make sure you full screen the video then lock your screen and you should be golden.

You can do this with any video website as well, not just youtube of course. 

 You can use the Zapstream PWA. It plays on my phone with the screen locked. Zapstream is using the same event as flare. Hence the content from flare displays on zapstream and vice versa.