SAP Fiori elements framework is based on SAPUI5 technology. You may be familiar with the Model View Controller concept in SAPUI5.
In the view, you can define the UI controls to be used in the UI.
The controller contains methods that define how models and views interact.
A model holds the data and provides methods to retrieve the data from the database and to set and update the data.
Thus, in a freestyle SAPUI5 app, you have to define the views and implement the methods needed in the controllers, for example, an event handler for a button if a user selects a button.
SAP Fiori elements framework generates views at runtime based on the predefined floorplans and your app-specific metadata and annotations. Thus, in the standard SAP Fiori elements applications, you do not have any static views or controllers in your project in the development environment.
If you use the extension points, a static xml fragment has to be added to the
webapp/ext folder of your project. If needed, a controller will also be added. The Page Editor can generate both files, including an
ext folder if not present yet.
In custom extensions, you can provide features that SAP Fiori elements does not offer, for example, process diagrams or maps.
Note: you can find the list of all extension points provided by SAP Fiori elements at Overview of Extension Points