EDIT: Just realized the substantial difference is that Google and Amazon own all of these things. They don't control all makes and models of phones.
1.) They have an economic incentive to do so
2.) Only they know how their systems work
3.) Their network traffic is encrypted
4.) They face legal risks and user backlash if caught doing so
Given 2+3, you can't be entirely sure that they aren't doing it. If they deny it, your only recourse is to hope that their cost/benefit calculus considers 4.) to be more costly in terms of dollars.
It will recognize the Hey Siri, but give an error for everything else. Pretty sure "Okay Google" will behave the same way. Also if you turn on battery save on iOS it disables "Hey Siri".
It's a custom low-power chip they added on the 6S and later to allow a local tight loop that only listens for that utterance using a hardware-assisted neural net, and only activates the rest of the software stack if it detected it with reasonable confidence.
There's a nice blog post about it here from the apple engineers (see the "Two-pass detection" section): https://machinelearning.apple.com/2017/10/01/hey-siri.html