Inquiry about creating your own extension

[PyAntony]

Hi,

I have read this guide: https://quarkus.io/guides/building-my-first-extension. I see that BuildItems are actually not doing anything; they just carry information. Let's take for example the following piece of code from the documentation:

@BuildStep
ServletBuildItem createServlet() { 
    ServletBuildItem servletBuildItem = ServletBuildItem.builder("greeting-extension", GreetingExtensionServlet.class.getName())
      .addMapping("/greeting")
      .build(); 
    return servletBuildItem;
}

My question:

Where is the piece of code that is actually interpreting and executing something based on the type of the BuildItem; something like (pseudocode): if (buildItem.class == ServletBuildItem) do {...}. Can you please post the link of the code where this is taking place? All I see in the documentation is that Quarkus process it. Thanks.