Another one is https://mynoise.net/, which has a neat mode where the sliders will very slowly move on their own, so you get slowly adjusting soundscapes.
I don't know if it works offline though, and probably not when your phone is locked. They also published recordings on spotify though; some are an hour long, others you can probably download them and loop them though. https://open.spotify.com/artist/1gRJBUyCeihBrgcCtDdEfv?si=yL...