In Creatio there are two types of pages. Classic pages and Freedom UI pages. Freedom UI pages are the newer types, the application is slowly getting converted to Freedom UI (Classic pages are now considered legacy, the future of the application is Freedom UI).
Classic pages are based on Extjs, although you don't really need much, or any, Extjs knowledge. That's really just the framework that the pages themselves use. When you add code on the page, you don't really need to use any Extjs functionality. For Freedom UI pages things are different. The article I mentioned is for how to do this on a Freedom UI page. These pages are based on Angular, although you don't really need any Angular knowledge unless you are creating new components/remote modules. The Terrasoft usage is mainly for working in Classic UI pages, much of the functionality there is being replaced by the new Creatio DevKit SDK. The Terrasoft library is one created by Creatio (the company name used to be Terrasoft).