If I buy a SmartThings hub, can that communicate with the Google Home app on my phone? Or do I need the Google Home device too? (Confusing marketing…)
I’m also wondering, what protocol does ST use to talk to Google? I’m assuming there’s a single protocol that hides all the complexity of different radios, types of bulbs, etc., and ST just presents all attached things over a unified interface. (Also wondering, is this the same as the protocol between Hue and Google, or Hue and ST?)
It’s quite a bit simpler than that. Just like the Amazon echo, the Google Home uses the cloud to parse the voice input that it receives. For a home automation request it just converts that to a message format agreed-upon between SmartThings and the voice processor. It’s not completely clear yet how that message gets transferred over to SmartThings. With echo, it’s a cloud to cloud integration.
With Google home, one journalist wrote that weave was being used, but no one else has confirmed that yet. (Tech journalists do get these kinds of things wrong occasionally.)
So it could be that a person talks to google home, the audio is passed up to the Google cloud for processing, the result is passed back to google home, and then Google home passes the formatted device request over to the SmartThings hub via weave. But I doubt it.
I think it more likely that a person talks to google home, the audio is passed up to the Google cloud for processing, the result is sent cloud to cloud to SmartThings, and the SmartThings cloud passes the formatted device request down to the SmartThings hub just like all the other SmartThings cloud requests.
If somebody wants to test this, it would be easy enough – – just unplug the SmartThings hub from the Internet while leaving google home connected to the Internet and see if a Google Home request gets through. But I’ll bet a virtual pizza that it doesn’t. 
The Phillips hue bridge is an entirely different situation. They have an open API, so any device which is on the local area network can talk to it. That includes a SmartThings hub, Google home, Amazon echo, Harmony, wink, etc. The protocol there is LAN. That will run completely without the Internet, but that’s because the hue bridge doesn’t have to do any cloud processing, unlike all of the voice processors so far.
Nest offers two means of integration. The first is its own version of thread, and that’s what most of the official integrations use. But because it allows for Internet access, there are unofficial integrations, including one developed by the community for SmartThings, which are cloud to cloud. Neither weave nor Thread is required between the third-party device and nest because the message is just getting passed in the same way that a request from an app would be.
So to get back to your original question, when you have a Google home and you have a SmartThings hub and you have set them up to talk to each other you can then control some of your devices (not all) by using voice to the Google home which then passes the message over to the SmartThings hub, Then SmartThings uses any of its normal control methods to perform the request. So google home is essentially just a voice UI FromSmartThings’ point of view.
Or you can still use all your normal methods through SmartThings for controlling compatible devices.
There’s a very active current community discussion thread about Google home if you would like to talk with people who already have both devices set up or if you have more specific questions. 
I don’t think Google’s cloud would talk to anyone else’s cloud, and sending the message over LAN seems more plausible, but maybe your guess is as good as mine :).
LAN is the medium, but what is the name of the protocol(s)? Google’s Weave could be one of those, but there could be others that’s more native to ST or Hue.
I’m still wondering if I need to Google Home device, or the app alone would suffice. I might head over to that other topic you mentioned, but I’m afraid the folks there already have the device so may not know the answer :).
Good point. Hue is simple:
This is a RESTful interface over HTTP
The message format is just Hue’s own open API.
Google has just recently opened their developers website for the Google home. What echo call skills they call “actions.” And they definitely offer a cloud to cloud option for web services
That’s pretty common these days. It doesn’t mean they get access to the entire cloud service, it just means there is a cloud-based access point for specific transactions.
Google can definitely do cloud to cloud and so can AWS.
Is the Nest integration still possible with Smartthings or has that been shut down? I have a few Protects and thermo I would love to connect if only for status.
Thanks. Wish this was just a standard smartapp provided by Smartthings!!!
Why not get the Lenovo Smart Display instead? It can connect directly with other Google devices you have. It’s basically a Google Assistant with a built in screen.