Google Assistant is one of the headlining features of the Pixel phones. There are a couple of different ways to use Assistant. First, set it up.
- Long-press the home button or say "Ok Google"
- Tap Turn on when Assistant pops up
- You will be guided through the setup
You can choose talk or type to be the default method.
- Long-press the home button.
- Tap the blue menu icon in the top corner
- Select Settings > Phone > Preferred input
- Choose "Voice" or "Keyboard"