What is a poly line in SU terms and how do you create one?
In General, ‘lines’ do not exist in SketchUp, only infinite guidelines. In stead, SketchUp has (finite) Edges.
They can be welded (‘stitched’?) by extension into Curves.
You can draw curves with the freehand tool.
Holding Shift creates a Polyline.
Polylines do not create ‘inference-points’ or faces. They are a bit ‘Un’ SketchUp, for that matter and it’s usability within SketchUp is not clear to me, I think of it as a rural leftover from the early days.