There are three volume settings in iOS, and they are easily confused.
Ringer mute. This is the slide switch. Although this switch typically mutes sounds from apps, it is possible for apps to override it. Which can be really aggravating when you find some app (typically an in-app advertisement) decide to do so.
Ringer volume. This is what you see on the “Sounds” preference panel of settings at the top of the “Ringer and Alerts” section.
Volume for everything else (app sounds). This is the slider you see in the control center.
The volume buttons on the side of the phone will adjust either the ringer volume or the app sound volume, but not both. The “Change with buttons” setting under the ringer volume slider is used to determine which it affects.
In your case, you’ve got your ringer/alert volume set to 50% and your app volume set to 0 (muted). And your phone’s side buttons are configured to change the ringer volume, not the app volume.
If you want to change your app volume, use the slider in the Control Center or disable the “Change with Buttons” option and use the side buttons.
FWIW, I usually configure my devices this way:
- Ringer volume at 100%
- Change with Buttons disabled (so the side buttons affect app volume)
If I need to mute the ringer (rarely), I’ll use the slider. I frequently change app volume using the side buttons, often leaving it muted.