Android
Set a Custom Notification Sound for Individual Apps auf Android
Android applies one default notification tone to every app unless you change it manually. Some apps also split notifications into categories, such as messages, reminders, or promotions, and each category can carry its own sound.
This granularity lives inside the notification settings of the individual app, not in the phone-wide sound menu. That is why changing the general ringtone in Settings ➔ Sound has no effect on apps you already customized.
Menu path to change the sound for one app
Open Settings ➔ Notifications ➔ App notifications ➔ select the app you want to adjust. If the app uses categories, tap the category that should get a new tone, then tap Sound. Pick a tone from the list and confirm with Save.
If the app shows no categories, the sound option applies to all of its notifications at once.
Some apps manage their own sound settings inside the app itself instead of exposing them to the system. In that case, check the app's own notification or sound preferences before assuming the feature is missing on Android.
Silencing a specific category this way is different from muting an app entirely, since muted apps stop showing notifications altogether.