To apply a render attribute, for example, class, you would provide the attribute and its desired value in the render attribute section of the field/element to add the attribute and save your item.
Then you would see your render value on your field/element:
This is greatly variable depending what libraries are loaded in your particular theme/bundle. Render attributes are all about display and leveraging available options. Those available/valid attributes depend on what is available to your forms when they render. (etc)