Whether you are a beginner or a professional, you can learn from video tutorials, self-paced courses, blogs and trainers on SketchUp's resources page. I edited the slightly and moved your post to the Free category.SketchUp Pro is a powerful 3D design software that lets you create, visualize, customize and communicate your ideas.
Make does support extensions and it’s still free, just a little older. Free does not support extensions, being Web based. If you want them use SketchUp Make 2017 which is free and can be downloaded here.
