You’ve got excellent live documentation of all sorts: your YouTubes; your "your topic is similar to…; ", and the drawing tips sidebar that I dismissed and can’t find again, etc.
However there are things I’d like to check out just to reference them directly (so that I don’t have to remember…now what video was that in?)
and
to compare SketchIUp to other CAD programs I’ve been exposed to with features like (multiple) viewports; (multiple) User Coordinate Systems; etc.
Is there a document (.pdf?) that explains SketchUp’s functionality or a url that is purely documentation?
there isn’t a User coordinate system as such. You can choose units and display precision in Model Info>Units that is appropriate to the kind of modeling you are doing. You can use coordinates in the model space but it’s generally easier not to use coordinates at all.
There are a variety of books out there such as the SketchUp for Dummies book if you want something in print.
Changing the SU Axis to whatever orientation one desires is pretty much the equivalent of changing the AutoCAD UCS. Of course, if he wants Acad’s dynamic UCS option where it “helpfully” reorients the UCS to whatever face you hover over, SU (IMO, thankfully) doesn’t have that.
This comes up every once in awhile from a new user who hasn’t spent any time actually modeling in SketchUp. Learn to use the tools as they are and then decide. I’ve used programs with multiple viewports along with SketchUp. I much prefer the single viewport in SketchUp. If you really think you have to have multiple viewports, start with a component, make copies and rotate them to give you the different views.