I am used to this with JQuery but i am unsure whether this is supported with EXT JS;
I presume the same thing happens as with JQUERY, I insert a DIV and name it with an ID and have it replaced with widgets/controls from the external JS file?
All the js should be in external files and you can build all the HTML via Ext if you want to - that is start with an HTML document that has nothing but the HEAD tag which includes Ext files plus your file and an empty BODY tag.
Suggest you start with examples. Many of which have nothing more than some static text in the BODY.