It's not standalone, it integrates into the closed Nest ecosystem (Google) of cameras, smoke alarms, thermostats etc. https://nest.com/uk/
I have most things integrated through Comfort although the UI side is now moving so fast, products like Nest provide an extremely slick user experience.
I was wondering if it were possible to integrate a Nest doorbell into Comfort by using the bell wires. The integration would be that Comfort would know the bell had been pressed allowing responses to be triggered etc. but the camera audio side would be handled by Nest. Probably over-complicating things and Nest looks to be a completely closed system.