Not having an iPhone: how do you access this side navigation? I don't immediately see any clues in the app. Is it the buttons near the top (three horizontal lines)?

That being said, it kind of reminds me of Windows Phone's swipe to the side. Windows Phone does give clues that there's more to the page by cutting off part of the edge. This is the default design language for Windows Phone.

On the iPhone, it almost looks like there's a built-in or popular library these developers are using to enable this. Any word on how this is accomplished?