This question is a little bit confusing as presented so let me see if I can clear something up.
Hub optional devices
Smartthings can be used in a “hub optional” configuration without using a hub. In that case all of the integrations will be cloud to cloud. Since the end devices will need to have some way of communicating with their own cloud, they will be either WiFi or something that has a Wi-Fi bridge, such as Lutron Caseta. So the device communicates to its cloud and then its cloud communicates to the SmartThings cloud. Or the smartthings mobile app will communicate to the SmartThings cloud which will then communicate to the device’s cloud which will then communicate to the device.
OK, those are the hub optional devices.
Hub connected devices
The smartthings hub is essentially a plastic box with several different radios inside. There is a zigbee radio, a Z wave radio, A Bluetooth radio which is not enabled (long story), And then some models there is a Wi-Fi radio. However, it is important to note that the hub will not normally communicate directly with Wi-Fi devices over Wi-Fi. Most will be cloud to cloud. But there are a few, fewer than half a dozen, devices which do have a direct LAN connection, such as the Hue bridge. And in most cases these specific WiFi Devices can be set up to either communicate cloud cloud (typically for the hub optional configuration) or locally. But all the other Wi-Fi devices have to operate cloud to cloud.
Directly connected devices that still have a cloud component
Ok, so zwave and Zigbee devices usually connect directly to the hub, But every device needs a “device type handler” (DTH) which is a bit of code that tells smartthings how to format messages. And all custom code, including custom DTHs, run in the smartthings cloud. It’s just the way the system is designed. So unless the code is created by smartthings itself and distributed to all customers (these are called “stock“ DTHs), The DTH will run in the cloud. So the device can send a message directly to the smartthings hub, but the hub has to pass it up to the smartthings cloud in order to know what to do with it.
Directly connected devices that operate locally
If you do have a device that can use one of the stock DTHs, it is then eligible to run locally.
Smartthings offers a web interface to your smartthings account. This is called the IDE and let you see some of the details of your account that are not available in the app, including whether a device is scheduled to operate locally or in the ST cloud or via a cloud to cloud integration. Devices using the new platform with a cloud to cloud integration will be marked as “placeholder.“
The smartthings cloud is actually divided into different “shards,“ so always use the universal URL:
Some people don’t understand the issue with multiple shards and may give you a URL that has a “graph.api.” in it or One of the other regional URLs, but just use the universal one and it will always take you to the right place for your own account.
Where Automations Run
At the present time, the only automations which are eligible to run locally are ones created with the official SmartLighting feature and some bits of smartthings home monitor. That’s it. Any automations created with the “add automation” feature or any custom smartapps or any Webcore piston will run in the smartthings cloud. Even the SmartThings mobile app will require access to the cloud just to turn devices on and off.(Again, they didn’t have to do it that way, but they did.)
So basically you as an individual customer don’t get to make the kind of choices you were discussing. Anything you do that’s custom is going to run in the cloud. Most Wi-Fi devices will be using cloud to cloud integrations.
If you only choose zwave, zigbee, and one of the few Wi-Fi devices which can use stock DTHs and you only create automations using the official smartlighting feature, those Automations will run locally. Otherwise, there will be a cloud component.
So if I understand your question, just using Ifttt with a particular device will not turn it into a cloud-based device. But that particular automation will obviously be cloud-based.
Whether something is cloud or local is on an automation by automation basis and depends on both the DTHs required and the automation itself.
Oh, and mode changes are always done in the cloud so if you use mode as either a trigger or a target of the automation, that will be cloud-based.