Starting with WordPress 5.9 a new Full Site Editor was introduced and is available on sites using block-based themes. The Full Site Editor allows users to create and theme templates using the drag and drop Gutenberg block editor. WordPress core introduced a very large number of placeholder blocks, available to all users, for standard site and post features that account for fields such as site logo, comments, as well as post title, content, featured image and so on.
Full Site Editing requires a theme that supports it. All block-based themes support the Full Site Editor. In theory regular “Classic” themes could also enable the Site Editor. Themes that do this are called “hybrid” themes.