"hotword" detection is done in special recognition hardware so as to not incur latency to the cloud, which is where the rest of the voice recognition gets done. You can't change the hotword, or can only choose between a few, because that's what's baked into the chip on the device.

Yeah, only problem is you could in fact, change it.

And then they removed that feature, long before it ran on a separate low power IC.

That's for Android phones. I'm actually not sure they run it from a separate IC anyway, like the iPhones.