Greetings, jack_h00.
Welcome to the Apple Support Communities. We can understand why you’d want your AirPods to be the selected input. However, looking at the GarageBand User Guide, using a Bluetooth device is not mentioned. Connect a microphone to use with GarageBand on Mac
"You can connect a microphone to your computer to record your voice, an instrument, or any other sound to an audio track in GarageBand. You can connect a microphone to an audio input port, USB port, or other port on your computer, or to an audio interface connected to your computer. You can also use your computer’s built-in microphone to record sound."
With that being said, you can try to reset your AirPods, save your info and close GarageBand, restart your Mac and then try again.
How to reset your AirPods and AirPods Pro
Log out, sleep, restart, or shut down your Mac
If the AirPods are still not an audio option, we'd recommend contacting Apple Support, to verify compatibility.
Get Support
Select Music, Apps & Services > Mac apps > GarageBand
We hope this helps! Don’t hesitate to reach back out with questions.
Best wishes.