For optimal performance and to make the experience as seamless as possible, the default setting is for the active microphone to switch automatically between one AirPod and the other. And if you wear only one AirPod, then that one becomes the active microphone.
If you want, you can change these settings so that either one or the other AirPod is always the active microphone. Before you change the setting, bear in mind that the AirPod you select as the microphone will continue to be set that way even if you remove it or insert it back in its case.
How to Set One AirPod as the Active Microphone
Before continuing, make sure you're wearing the AirPods and they're connected to your iOS device.