She recently released a course to help people learn about Full Site Editing. The course is still under development, but worth taking to become familiar with what the full site editing project is all about. It is an overview created for theme authors who wants to learn about full site editing, but also good for those building block editor based sites using commercial themes.
The goal of the full site editing project is to utilize the power of Gutenberg’s block model in an editing experience beyond post or page content. In other words, the idea is to make the entire site customizable. This editing mode will understand the structure of the site and provide ways to modify global elements like headers and footers. You can read more about it here.
Here is the intro video from the course site: