A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
By "site name" where exactly do you mean? You can add a logo image in the top bar or you can add a background banner image in the site title area. There is a text web part that can be added to a page content area and you can insert images into it. If you really want to add your own html/css/js, you should look into the modern script editor web part.