You used to have to dig through your Google Account settings to get access to the settings for Assistant. But that's about to change. The latest beta version of the Google app (version 10.53.3) will make it easier to access Assistant's settings. Open Settings in the app and go to Apps & notifications. There are toggles for pulling up Google Assistant (either by a Hey Google call-out or through squeezing the sides of the phone). Below that is a one-tap link to all Assistant options in the Google Account settings. If you're a Pixel user, expect this to head your way soon.
Source: Android Police