Almost all Android devices have Google Assistant turned on by default. To check if it is active on your device, long press the home button. If it is active, the assistant will start listening to you; If not, you will be asked to turn it on by a small prompt at the bottom of your screen.

g.co/assistant/setup If the prompt doesn't appear, you can turn it on in the Google Assistant settings like this:

Launch the Google app.
Tap on your profile picture and select Settings.
Go to Google Assistant > General.
Enable the Google Assistant toggle.
When turned on, your Google Assistant is ready to answer your questions, follow your commands, and show you relevant search results. Just press and hold the home button and speak. If you prefer gesture navigation instead of buttons, swipe your finger diagonally from either side of the bottom corner of the screen toward the center.