how to manage volume on airpod 4?

See this :

• xTo turn the volume up or down, say "Hey Siri," then say something like "Turn up the volume" or "Turn down the volume." Or drag the volume slider in the app that you're listening to, on the Lock Screen of your iPhone or iPad, or in Control Center. You can also open Now Playing on your Apple Watch and turn the Digital Crown to adjust the volume of your AirPods Pro or AirPods 3 or later.

Or adjust the volume on your iPhone with the volume sliders