In-call audio. When you’re on a call, the Volume buttons control the volume of the other person.
Media. When you’re not on a call, the Volume buttons control media volume during playback or before you listen.
Captions for all speech. Tap to turn Live Caption on 

Phone ringtone, notification sounds, and alarms. To adjust these volumes, press a Volume button and then use this shortcut on your home screen:






You can adjust the volume of each app.
Also, your phone can learn to automatically mute apps you usually mute. You can review and adjust the list of apps at any time.

Press a Volume button to control volume of the app currently on screen, or use the sliders to adjust volumes of any apps with sound in use.

If you don’t want your phone to learn your usage patterns and automatically mute apps, go to Settings > Sound & vibration > Multi-volume and turn Auto mute off 
To silence a ringing phone, press either the Power button or the Volume down button.
You can simply pick up your phone to silence incoming calls and alarms, and to turn vibration on.
To enable this gesture: