Apple restricts usage of non-Siri voice assistants on its iOS devices, but Google has figured out a workaround.
To get Google Assistant to work on an iPhone, do this:
 Download the Google Assistant app.
 Open Google Assistant on your iPhone.
 Tap the Add to Siri option.
 As part of the setup process, the Google Assistant app will ask you to record the phrase “OK Google”
Now, to access the Google Assistant, just say “Hey Siri, OK Google” if you have always-listening mode enabled for Siri.
Maybe Amazon will get wise and try the same thing soon.