As your GUI evolves from design into the implementation phase, you will need to integrate code generated by the Interface Builder with your own code to produce a working application.
At one extreme, you can attempt to specify the entire GUI for an application using the Interface Builder: even callbacks, keyboard accelerators for menu items, and so on. This way the source code for the entire GUI would be generated automatically. However, this is not the recommended approach.
Instead you should use the Interface Builder for the basic design and initial code generation only. Once you have created an interface skeleton for your window or dialog that you are happy with, augment the automatically-generated source code with hand-written code. At this stage, you will use the Editor tool, rather than the Interface Builder, to develop that window or dialog.