Earlier, SketchUp used to provide a free desktop software edition with limitations for Windows PC. However, it now provides only a free web app that you can use to view and edit SKP files in a browser freely. The SketchUp software editions for PC are now commercial and subscription-based. So, the desktop users will have to purchase a subscription and then they will be able to download and use SketchUp software on their PCs. You can check out the details on its free web app which has some feature limitations in this post.