In the link that Winston put are tutorials about custom drawing, how canvas layers work and other useful stuff too.
You can also use Line2D nodes which can make fancy lines on top of Polygon2D nodes (using the same set of points), there are MeshInstance2D nodes too but are a bit hard to use.
And Godot importer can rasterize SVG files (in editor, not at runtime) if you want to just use svg assets.