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 respond.
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.
I’ve found this works great, I’ve also tried
Hey Google with multiple devices and saw some not so great results as all devices rush to answer at the same time and respond to your request.