Google Assistant ---No need to day Hello and Hi for every query...

You’ll still need to activate your Assistant devices with “Ok Google,” “Hey Google,” or with a physical trigger, but with this new optional setting turned on, the Assistant will stay active for long enough to respond to follow up questions so you don't have to say “Hey Google” as often," explains Google.
In order to end the conversation, users will have to say, “thank you” or “stop.” The conversation also automatically ends if it detects that the user is not talking to it anymore.
How to enable Continued Conversation in Google Assistant app
To enable this new feature, go Google Assistant app by going to Settings > Preferences > Continued Conversation and hitting the toggle.