To adjust the volume level of all notifications:

Sometimes, you get a sudden influx of notifications - when everyone on a thread likes a comment or when you turn off Airplane mode after a flight. Notification cooldown can help. When on, if you receive many notifications in quick succession from the same app, Notification cooldown lowers the volume and vibration, making them less intrusive.
Calls, alarms, and priority conversations are not affected.
You can modify the notification sound for all apps, or assign a distinctive sound to notifications from a specific app, replacing the default.
Do one of the following:

Tap the category for enabled notifications, then tap Sound and select a unique ringtone, or set it to none.
If an app's notifications are silent but you want to hear them:
If you're only getting some notifications from an app, check if Notification cooldown is on.
If you want an app's notifications to be less intrusive, you can categorize them:
Option 1: On a notification
Option 2: In Settings
To hide the icons for silent notifications from all apps:

Restart the phone to apply the new settings.
You won't see the icon in the status bar for any silent notifications. When you swipe the status bar down, the icon appears in the pull-down along with the notification.