Make WordPress Core: Introducing theme.json in WordPress 5.8
mkaz.blog: Using theme.json in a classic theme
Building Modern WordPress Websites (Rich Tabor @ Wordcamp US)
Really nice introduction to block themes, theme.json, site editor and all those kind of modern WordPress stuff.
He’s also writing some very interesting technical posts on theme.json on his blog.
Anders Norén: A New Era for WordPress Themes
ThemeShaper on Full-site Editing
ThemeShaper is also doing a very interesting series of post on Full-site Editing, including a nice introduction to get started, templates, global styles, etc.
Finally, the Blockbase theme it’s supposed to be a good starting point for new block theme, but I’m not quite sure how it compares to the empty theme experiment (published on the WordPress official Github repo)