To select AirPods as the audio device on your iPhone, follow these steps:
Open the Control Center on your iPhone.
Press and hold the audio card in the top-right corner of the Control Center.
Tap the AirPlay button.
Select your AirPods from the list of available devices.
If your AirPods are not showing up in the list of available devices, make sure that they are properly connected to your device. If they are not, try disconnecting and reconnecting them.