I used this site to help implement some core utils in TypeScript for Deno. They never had all the features, but it helped understand the code paths, etc.
I frequently "info coreutils". I poke around and almost always learn something new or refresh something that had faded. Sort of a playground for me...
I have learnt a lot from Maizure's blog. One of my early favorites: <a href="http://www.maizure.org/projects/printf/index.html" rel="nofollow">http://www.maizure.org/projects/printf/index.html</a>