Boards and sensors that are compatible with DK IoT Studio can be found at the DK IoT Studio Components page.
The component page also includes optoelectronics and connectivity boards as you scroll down. From this page you can also view the components main page on Digi-key.
At the product main page you can view datasheets, product specs and associated product. A list of these components can also be found within DK IoT Studio when you make new projects. This lists change as new boards and sensors are made compatible with the Studio.