If we consider the way these online documents work "behind the scenes", that is, the functioning of the backend software that Google has put in place to allow real-time editing and sharing, we shouldn't be surprised that a part of it has become available to Apps Script programmers.
Remember that the file we are about to manipulate will not be hosted on our computer, but somewhere on Google's servers. The page we actually see is nothing but the rendering of a set of instructions that we sent to this server. Each keystroke, paragraph, or format we change may be considered as a small code sent to the server running an application.
Of course, we don't need to know all of them. The engineers who designed the Google Docs service API had to translate it in a JavaScript-compliant language that anyone could use.
A look at the available methods at https://developers.google.com/apps-script/reference/document will tell you more about the plethora of methods than a description in this book...
