All objects that will be rendered on the screen (and sometimes not rendered, but presents invisibly) are called components.
The source code of all components is located in the folder src/cljs/webchange/interpreter/renderer/scene/components/.
There is a set of ready components that you can use in your activities. The list of ready-to-use components can be found here.
Also, you can create your own component and use it. Here is how to create a new component.