I use a second account just for it

they're free to steal it if they want

I'm pretty sure it's modded version of the official youtube app so no guarantees (Just use a dummy account?).

I'm actually very interested in how the vanced developer has made this. This app is pretty much identical to the youtube app with premium features enabled and ad/sponsor bocking/skipping.

I never log into YT on any device. I still get great recommendations and easily stay on top of my favorite channels. IMO the only benefit to logging in is being able to comment.