how to use HomePod mini as a mic for Mac mini
It cannot be used as a microphone and DOES NOT show up under input in the sound settings.

https://reddit.com/r/macmini/comments/k1b271/limitations_of_using_a_homepod_or_homepod_mini_as/gdngtrm
It acts as an AirPlay receiver like Apple TV, not a Bluetooth device like AirPods.