Hi Pixsa666
Welcome to the Apple (user-to-user) Community.
Apple support's article (link below)
Adjust the volume on iPhone – Apple Support
says:
"When you’re on the phone or listening to songs, movies, or other media on iPhone,
you can use the buttons on the side of your device to adjust the audio volume.
Otherwise, the buttons control the volume for the ringer, alerts, and other sound effects.
You can also use Siri to turn the volume up or down."
Also:
"When iPhone is locked or when you’re using an app,
you can adjust the volume in Control Center.
Open Control Center, then drag "
Hope this helps:.