Correct, all communication is done via the cloud right now. Google does have a local exec API, but it's not currently used by GHC.
GHC also never "pushes" any data to Google; it only responds when Google asks for something. That means that for something like a smoke detector it won't be able to push alerts to you. Again, Google has an API for that, but it's not implemented due to Hubitat platform limitations. @bhamiltoncx created a feature request for what we would need, and the Hubitat devs didn't seem to be against it, but there's not been a concrete answer yet.
