Not how it works.

They have an ultra low power processor that listens for the "hey Google" keyword, then that wakes up the main audio processor. But the main microphone is not actually on, and that small processor isn't capable of recording audio it's just looking for a certain matching sound wave and then triggering.

That's why it sometimes triggers if you just go "hurr ner dorrll" because those random sounds are close enough to what it's looking for.

That is why some older devices can't actually install the assistant software. They lack the necessary hardware to do it in a power efficient manner.