Hidden classes are exactly as they read: hidden. You don't have access to the underlying code-base for these classes. Any rewrites to these classes are the scripter's best interpretation of how the script works (usually) in pure Ruby. The Tilemap class, for example, is most likely largely written in C code since the amount of drawing it does is performance heavy.
btw Selwyn's Window class rewrite is not a perfect copy of Window.
Blizzard's is nearly an exact replica where Viewports are not necessarily used correctly, but no one should be accessing a Window's viewport to begin with. Try using this windowskin and compare the two scripts
You should see the difference at the title screen, namely the cursor.
That script you linked from the ARC dev thread just grabs a list of the classes' methods and variables. Again, there's no actual source code because
it's hidden. This was used mainly to get an idea of what they needed to implement their own version of the hidden classes.
For getting the fonts, it probably just uses the Windows' environment variables. For example, in Windows Explorer, typing
%windir%\Fonts will bring you to the fonts folder. If the font isn't found, then RMXP just doesn't draw anything (I mean, there's nothing wrong with that). There isn't a way to manipulate the underlying code to look elsewhere for font files, so you have to use
a script to get that done.