You can start building elements in the Plugin Editor, clicking on Elements. You can find a documentation for creating plugins here, and in the Plugin Editor itself. A contextual reference will show you how the API works and to which objects you have access when you’re coding an element. The general spirit is that you start by defining the different fields users will need to fill in the Property Editor in edit mode, the events and the actions that apply to your element, and then start implementing the code.
Elements can trigger workflows, have some actions modify their behavior, and expose some data to other elements. In other words, you can now build an entirely new UX element for your apps (or others’ apps).
You can see a read-only, very simple example of a plugin here.
As usual, let us know how this goes!