Saying "Hey Google" before each follow-up question for the Assistant doesn't feel as natural. Google announced Continued Conversation at I/O as an optional setting which lets you have a natural back-and-forth conversation with the Assistant without repeating "Hey Google" for each follow-up request. The new feature is starting to roll out today, and you can turn it on in the Google Assistant app by going to Settings - Preferences - Continued Conversation and hitting the toggle.
With Continued Conversation turned on, after you ask the Assistant a question, you can ask a follow-up, set a reminder, or add something to your shopping list without having to say "Hey Google" each time. You can say "thank you" or "stop" once you're done, or the conversation will once the Assistants detect that you're no longer talking to it. 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.