In this case, it's a library named wetcontrols but each controls aren't name with this namespace. They are the prefix 'Wet' because it's an extension of the .net controls. WetTextbox extends Textbox but it's the same thing when you call a control TextboxExt.
You can easily build your controls or extend control like textbox by adding properties, methods or changing the render of it. The best way to understand how to do that is by exploring my custom controls on this github project:
If you like...