Once you have named and opened your app, you will get a screen where Qlik Sense asks you to load your data:
When you have this option before you, you can load data in several different ways. The Add data command to the left will start with a wizard that helps you define what you want to load. In the background, it adds code to a script that defines the load sequence. It will, however, never show the script.
This is different from Data load editor that will take you to a script editor, where you can change the script directly.
The easiest way is to use Add data dialog to the left. This will open the Add data