Assistant has had native support for security cameras for a while. Unfortunately, these were limited to simply streaming the live feed to a smart display or a Chromecast. Even though most doorbells could be configured as security cameras, they couldn't notify you when someone was at the door using Assistant. Google just updated its digital helper, allowing smart doorbells to send notifications natively.
Smart doorbell manufacturers can now configure their devices with the right type, making them appear as they should in the Google Home app. This also means that they now have the ability to send notifications to Assistant-enabled devices, in addition to playing a live video feed of what's in front of your door.
If you've already enabled notifications using your doorbell's app, this new feature will simply mean you'll now be able to be notified on your smart display when someone's at your door and see who's outside. Interestingly, Google hasn't extended the feature to cameras yet, meaning only doorbells can send notifications natively for the time being. Also, it will be up to the developers to define whether the notification is triggered when presence is detected or when someone actually rings the bell.