SmartThings supports several different protocols. The hub itself actually doesn’t have a Wi-Fi radio at all, it connects via ethernet connector cable to your regular router.
The SmartThings hub is a white plastic box that contains four different communication devices. There is a Z wave controller, zigbee coordinator, the ethernet connector, and an inactive Bluetooth radio.
And a device, like a motion sensor, might use any one of these protocols. If it is a Z wave device, it will communicate directly with the Z wave controller in the hub. It does not use Wi-Fi.
If it is a zigbee device, specifically if it uses the zigbee home automation profile (ZHA 1.2). It will communicate directly with the zigbee coordinator in the hub. Again, Wi-Fi is not involved.
If it is a Wi-Fi device, it might communicate with the hub over your local area network by going through your router, or it might communicate with the hub “cloud to cloud” where it talks to its own account over the Internet, that account sends information to the SmartThings cloud, and the SmartThings cloud communicates back to the hub at your house Via the internet and the hub’s Ethernet connection.
You can Read more about the protocols that SmartThings supports in the community – created wiki FAQ:
As far as your guesthouse, I suggest you read the wiki article on how to automate an outbuilding. It should give you some ideas.
Also, the stronger your Wi-Fi is, the more difficult it will be for your zigbee devices to communicate because wifi can drown out zigbee. ( Z wave is on a different band and is unaffected.) so if you have a lot of boosted Wi-Fi on your property, you may find that the Z wave motion sensors work better.