Here is a way to access Google Assistant תחילת העבודה עם מענן לענן | Cloud-to-cloud | Google Home Developers using an Agent and Android Studio. I don't think you can do that with App Inventor. Note this warning: Warning: You can't launch commercial devices that integrate with the Google Assistant SDK. It's available for experimental and non-commercial uses only.
Consider what you can do instead:
or You might find this useful HOW TO: Program the native SpeechRecognizer for Continuous Dictation and to do things
and a similar discussion תחילת העבודה עם מענן לענן | Cloud-to-cloud | Google Home Developers