Gtk Tutorial
When learning GUI frameworks like GTK, QT and wxWidgets it is common to teach the basics using code to make the learning experience easier. In a real life GTK application you will probably want to create a ui file using either GTK Builder XML or Blueprint to build the UI.
On the internet you will often see Glade mentioned for building GUIs in a user friendly wysiwyg editor. Glade does not work with Gtk 4 however there is a new project Cambalache which will build GUIs in a similar way to the old Glade project that was used in GTK 2/3.
If you want to use Blueprint to build your UI then you can use Workbench to see a preview as you write the blueprint.