Apple's WWDC 2021 keynote is nearly two hours long, because Apple has a lot in common with Zack Snyder. Nestled in the presentation on HomeKit was an interesting nugget of information: Siri will be available on third-party devices later this year.
That's a little less exciting than it sounds. It's not as if you'll be able to install a Siri app on Windows or Android: Apple's only extending this functionality to devices that are compatible with its HomeKit smart home technology. During the keynote, Apple demonstrated a Siri voice command working with an Ecobee thermostat (which indeed, includes its own microphone for voice commands).
Third-party smart home appliance manufacturers will be able to integrate Siri commands into their HomeKit functionality later this year, assuming they have the hardware to support it. But you may need a HomePod or HomePod Mini in order to use it: the presenter said that commands go to your HomePod over your home's network without going to a third-party server. Based on the demo, it looks like it should be possible to add Siri voice command functionality via a software update, so those who want it won't necessarily have to buy all-new smart home gadgets.