I have several iPhones / iPads, and when I say "Hey Siri what's the weather tomorrow", only my main phone will respond, how is it designed? 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:

https://support.apple.com/en-us/HT208472