Not sure any examples have been “publicly” posted yet, but try installing the various extensions yourself. Then you can open up your browser dev tools and look at the received html/js/css files the extension loads. Obviously you won’t be able to know how their backends work if they use one, but it’s a good start. You can copy the files to your computer and unminify if necessary. Some extensions are harder to look at because of their build tools, but others are quite vanilla and a good read. I’m working on a simple extension right now myself and will definitely post it when it’s finished.