If you want to quickly activate it, you can add Background Sounds to your Control Centre (which you can access and customise by swiping down from the top-right corner on an iPhone with Face ID, or up from the bottom on a Home Button model).
Alternatively, you can go Settings > Accessibility > Accessibility Shortcut and choose Background Sounds.
First up is Background Sounds, which is buried deep inside your settings app, but extremely handy for getting to sleep.
Then you can turn Background Sounds on or off just by triple-clicking your iPhone's Side Button (the one that locks the phone).
You'll find the feature in Settings > Accessibility > Audio & Visual > Background Sounds.
