All of it in service of one thing: showing you a set of documents you own but have never seen together.
The short version is here. What it reads and how the tree and the page work each have a page of their own with the whole of it.
github.com/owner/repository, the short owner/repository or a page inside the repository such as its releases. Anything that names no repository is refused with the reason before GitHub is asked anything.Markdown, plain text, HTML, Word and PDF beneath the folder you point it at; a single document opens from the second button.
The page covers what each kind becomes on the way to the pane, why opening a folder does not read them and why nothing a document carries is run or fetched.
How the tree mirrors your folders: arrows, counts, what is hidden by default and the one button that shows it all again.
Then how the document you pick is read: its fields laid out plainly, a readable line length, code and diagrams drawn whole, the page reading itself down and the count at the foot of the window.