Unfortunately, Google’s approach to setting up external services is a little convoluted compared to the Alexa skill system. During setup you are prompted to add your accounts for media services like Netflix and Spotify. Now let’s say you want to setup your thermostats so that you can control those with Google Assistant, that’s not quite as straightforward.
To add your home automation devices using the Google Home app you have two paths. From the homepage you can tap on the Add button, select Set up device, then choose Have something already set up?, and from there you can choose your device provider. Alternatively, you can tap on the Settings button, scroll to the bottom and choose More settings, then from the Assistant tab you can select Home control. This last method also gives you access to the remaining options for Calendar, Shopping List, etc. under the Services tab.