Playground
This section isn't part of the theme. It exists to show what a site's own section looks like: a page, a widget, a menu item — and not a single edit to the theme.
The theme has no "playground" shortcode, no styles for it, no mentions of it at all. The whole section is three things on the site's side: this page bundle, widgets.js next to it (the widget mechanism works for any bundle, not just guides), and a menu entry in the config with params.badge = "β" — the theme's navigation draws the badge on the entry; that's a generic feature of it.
So the playground has something to show — a toy built on the theme's building blocks:
Any section of your own gets added the same way — a glossary, a catalog, a trainer: a page, widgets if you need them, a menu item. It's covered step by step in "Make it yours".