The SketchUp User Guide actually first begins on this page …
The page John B linked is the Help portal page. From that page if you open the “Help With…” drop down menu, it shows all the individual links to the various SketchUp family of products both desktop and online.
They are in a certain order which becomes more evident with use.
However, I find it (the Drupal system) to be difficult because the chapters are collapsed except for the current one. It is easier to find what you want when they all can be expanded at once. Also sections are not shown in the right side navigation list, you have to often open the first page of a chapter, and sometimes scroll down to see the section list of links.
The built-in search for the help has been complained about for years, with the oft answer that the Trimblers themselves often instead use Google to search their own help articles.
There have been several topic threads complaining about the help articles and system lately …
This is that “peek-a-boo” thing that pops out of the right side. Most ignore it, and there have been recent complaints about that as well in the Meta category.
SketchUp does have a built-in Instructor panel that will display help and feature instructions for all native tools and Ruby extension tools if the coder authored them. At the bottom of each instructor page there is usually a direct link into the online User Guide that should bring you to the article for using that tool.
In addition, at each state of the natives tools, the status bar displays concise help and modifier key usage.
BTW, this thread belongs in the Meta category.
(Thank you to @TheOnlyAaron for reassigning this to the correct forum category.)