The video from Apple Support, embedded below, tells you to tap the white side of an AirTag to the top center of your iPhone to read the NFC tag. Watch the video, it’s short.

My instinct, based purely on where the animation appears when I open my AirPods case, would be to tap the AirTag on the bottom of the iPhone. I do recognize that AirTags and AirPods use different flavors of wireless magic, found this interesting, worth passing along.