As stated by @jeremydouglass , and explained in the Using the Twine Cookbook page of the Cookbook, each recipe includes a Live Example that you can download and import into the Twine 2.x application. This allows you to read through the example’s source code to discover how the related effect/outcome was achieved.
In regards to Harlowe 3.x there are two recipes you can examine, one that uses the special
?Sidebar named hook (that only exists in Harlowe 3.x) to achieve the required outcome, and one that uses a re-positioned footer tagged Passage (that works for series 2.x and 3.x of Harlowe).
The one you linked to is the second of the above mentioned methods.
The reason it is more difficult than possibly expected to display dynamic information within the area of the screen where the default Left Side-bar appears is because the built-in ability to do so was only recently(ish) added, and that ability as such is limited in functionally