The state and events are there on the camera and the nest app. That is how you get notifications using the Nest app. In the Nest App you can delve into a lot of different states/events presented by the camera however these have not been enabled in the Google Home environment for routines.
I suspect this is the case for many of the devices, where only a very small subset of the actual API events are "whitelisted".
Frankly it is ridiculous. I can use automation within environments like Grid Connect, to trigger on motion and a bunch of other things, but not the Google devices. Sadly, because am very anti-Apple, it pains me to see Google is fast becoming like them. Instead of being the open innovator, it is turning into a very controlled environment. Look at Google Nest Max Hub: try showing a simple calender on there - no chance.
Anyway back on topic. There is a way if you use Google Cloud tools, set up a development environment, and then access the Nest API directly. From here you can create the routines you need to implement motion detection. That is the only way currently until GH whitelists those actions in their native Routines offering.
