You may not need to use a third-party relaxation app anymore. With iOS 15 and iPadOS 15 Apple has added soothing sounds built into the OS.
You can find these sounds in Settings > Accessibility and look for Audio & Visual in the Hearing section. The sounds include:
- Balanced Noise
- Bright Noise
- Dark Noise
Like any audio you can control the volume, but you can also choose to have it play at the same time as other media, or keep it separate. For quick access you can add the Hearing toggle in Settings > Control Center. When you long-press the icon you can enable/disable Background Sounds from there.