Use the Gemini app 
Use any of these shortcuts:
Say a command or ask a question.
Ask about your current screen by saying ‘What am I looking at?’
To learn about changing settings for 

Tap your profile icon 

If your phone isn't recognising ‘Hey Google’ commands:
Check Battery Saver: when on, voice triggers (‘Hey Google’) are disabled. Use a different shortcut to start voice commands or turn off Battery Saver.