HomePods are controlled via the Home app on iOS, which uses Apple's HomeKit. Someone wrote a Node.js service called Homebridge that lets you write your own plugins to show up in Home, so I did some coding and now have our Raspberry Pis that have the temperature sensors attached showing up right from within iOS! I'm going to write up a blog post going over how I got it all going in detail, because there was a LOT of trial and error.
