For a fun experiment, yell RED FIRE TRUCK into your phone then immediately activate Siri manually.

With Hey Siri turned on, you will reliably get all three words. With it off, you will usually get Chuck or Truck, or sometimes FIRETRUCK depending on timing.

This seems to be for people who start talking before the Siri prompt is fully active like “Set <button> Timer for 5 minutes”.

But shows that Apple is typically pre-recording if not also pre-processing.