For those unaware, iOS includes a limited selection of such sounds. You cant mix or anything, but it might be good enough, at least it is for me. Search for Background Sounds in settings.

Yes, and the same feature is available on Mac too. I occasionally use it with headphones for work, when I want to stay focused.

Their "dark noise" combined with airpods / headphones in noise cancelling mode is enough to put me to sleep in almost any environment, it's great.

Yup, came here to say that too. iOS Settings - Accessibility - Audio & Visual - Background Sounds. Offers the same as OP's app, except mixing. The only thing I miss is a a fade out possibility. For example, set timer at 01:00 minute and it slowly fades out over that whole minute. Much better than such an abrupt end. Not sure if OP's app does offer that.