ELI5 Voice Agents - How does “hey siri” respond if it isn’t constantly listening to my conversations?

How does siri hear me say “hey siri” if it isn’t constantly listening to my conversations or me speaking? Why are most voice assistants’ wake phrases 3 syllables? “Hey Siri”, “Alexa”, “Hey Google”. How are synthesized voices (like Alexa and Siri) made? Why did the guy who is the voice of Siri not even know his voice was being used?
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