Latest alexa's have offline voice control. Is that through zigbee only or can it be through http or other methods?

I like the alexa features and skills it provides, but it really is lacking in the local voice processing department. I’m hoping to find a workaround that will allow me to use alexa online and offline without having to resort to a second voice assistant like mycroft. I don’t know how the new alexas achieve offline voice processing so any clarification helps. If it is through zigbee only, then one idea would be to use a zigbee transmitter/receiver connected to a HA pi. Any ideas?
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