Components are grouped by the ID spaces. The page itself is an ID space. The window component is another ID space. Assume you have a page called P, the page have a window called A, and the window A has a child window B. Then, if you want to retrieve a child component, say C, in the window B. Then, you could do as follows.

comp.getDesktop().getPage("P").getFellow("A").getFellow("B").getFellow("C");

The getFellow method is used to retrieve any fellow in the same ID space. Refer to the ID Space section in the Basics chapter for the concept of ID spaces.