That’s not exactly how it all works, but we would be getting way off topic for this thread if we start going into protocol distinctions.
Why don’t you start by reading the following FAQ, and then if you still have questions about the protocols you can start a new thread under devices and we can talk about it there.
Meanwhile, as far as selecting devices that work with SmartThings, most people will be fine with either Z wave or zigbee for any given device, you just choose the one that has the best features for what you need.
You just need to make sure that you have enough repeaters overall of each protocol that you are using. See the following FAQ on those issues. Start with post 11 in that thread, then go back up to the top and read the whole thing.
