I've been working in the extension space on a variety of products for a number of years now and decided to put together a course on how everything I wish I knew when I first started out. It goes through building an entire "product", meaning UI, API, and extension, all communicating with each other. It covers a lot of topics I get asked about often as well such as extension-level authentication, injecting React apps into web pages via content scripts, and a bunch more.