When you say “Hey Siri” near multiple devices that support “Hey
Siri,” the devices quickly communicate to each other using
Bluetooth to determine which one should respond to the request.
The device that heard you best or was recently raised will
HomePod responds to most Siri requests, even if there are other
devices that support “Hey Siri” nearby. If you want to use Siri on
a specific device, raise to wake that device or press the button
to use Siri, then make your request.
Works pretty well (and very quickly) in my experience.