That's is, if I have several iPhones and iPads on the desk for lying there for an hour, and I just said, "Hey Siri..." without touching any of the devices, only the phone that I use most often will respond. Does Siri know which one is my main phone and only use it to respond?
When you say "Hey Siri", the devices that could possibly react to it will communicate with each other using Bluetooth to determine which device will respond: The device that heard you best, or the device that was most recently raised or used will respond to the "Hey Siri" request.
Apple has detailed the method here:
"Hey Siri" only really works on select newer iPhones. I had an iPhone 6 for a while and had the option for "Hey Siri" but it never worked. Upon getting an iPhone 7 plus... The "Hey Siri" feature works great! Your main phone must be newer than the others if "Hey Siri" only works on that one. That or "Hey Siri" is disabled on the other phones and/or iPads.