Note here the special line

This code is responsible for inserting the contents of each page into the layout. It’s not important to know exactly how this works; what matters is that using this layout ensures that, for example, visiting the page /static pages/home converts the contents of home.html.erb to HTML and then inserts it in place of <%= yield %>. It’s also worth noting that the default Rails layout includes several additional lines:

This code arranges to include the application stylesheet and JavaScript, which are part of the asset pipeline (Section 5.2.1), together with the Rails method csrf_meta_tags, which prevents cross-site request forgery (CSRF), a type of malicious web attack.

    Of course, the views in Listing 3.22, Listing 3.23, and Listing 3.24 are still filled with all the HTML structure included in the layout, so we have to remove it, leaving only the interior contents. The resulting cleaned-up views appear in Listing 3.26, Listing 3.27, and Listing 3.28.