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They actually have thought it out well. The short of it is you can set up a device to constantly stream to home assistant so that you can use any wake word. That comes with the draw back of more power use and more importantly higher cpu use for each microphone you add. It's still possible (https://github.com/dscripka/openWakeWord)

The alternative for a dedicated low power device is to have trained a model to run on smaller micro controllers so that it can run locally on low powered devices (https://github.com/kahrendt/microWakeWord) this is what they have chosen for their dedicated devices.

This choice also comes with much higher default privacy. Which is great as home assistant offers cloud integration and the fact that they put privacy first in this area makes it much easier to trust they do the right thing in other areas.

A brief run down on the subject https://www.home-assistant.io/voice_control/about_wake_word/