I don’t believe that’s what the OP is asking, and in addition, there are a number of smartthings brand devices which don’t require a hub, like the SmartThings Wi-Fi smart plug, the smart appliances, and the smart televisions. So as always, the first rule of home automation applies: “the model number matters.“
@shivpatel , can you be a little more specific?
Smartthings does publish an API as as @GSzabados mentioned, So you can definitely integrate cloud to cloud using that. For example, cloud to cloud is an option for integrating a Hue bridge to SmartThings.
But there are multiple different configurations and they have different answers to your questions
are you just asking about the smartthings brand Zigbee devices, such as the Zigbee 3.0 sensors? Those are certified to the Zigbee 3.0 standard and can be added to any certified Zigbee 3.0 hub.
are you asking about the smartthings brand Wi-Fi devices? Bringing them into your system? Those are trickier. You have to add them to a smartthings account and then use one of the cloud to cloud integration methods. MQTT is popular for this, but there are other options.
are you asking about having your system show up in the smartthings app? That is, the Zigbee devices are joined to your hub but you want them to show up in the smartthings app. That’s doable, as in the Phillips hue example, but you have to use the API and it usually ends up as a cloud to cloud connection.
So… what UI are you expecting to use? The SmartThings app, or your own? And what specific smartthings devices do you want to control through that UI? In what direction is the integration? Do you want to bring your devices into a smartthings account? Or do you want to bring devices connected to a smartthings account into your system?
So there are a lot of different ways to use smartthings with other systems without having a smartthings hub, but it all comes down to the details.