I’m using Musi (which is designed to stream songs off youtube). Behind the scenes, it likely uses mechanisms akin to popular youtube downloaders, and is finicky where age-restricted videos are concerned.

But with it, I’ve yet to see a single ad and it can play audio from youtube in the background.

It can find and play playlists but has a poor story of following channels. You cannot navigate to a channel with it and binge on videos from it.