Examples: type sleep noise into the android Play store and the top few. Waves, bird noises, rain etc. All useless to me - just want masking noise to block other stuff out.
I use so-called analog or mechanical sound machines like the Marpac or Yogasleep Dohm, so not much use for other noise generation stuff. But every now and then I use an app when I don't have my machines, which I sometimes travel with.