Articles
One idea per page, usually next to the thing it gets confused with. These are the words a system design answer is built from, and the ones interviewers poke at: what a token actually proves, why "Sign in with Google" is not OAuth, when a 403 should really be a 404.
Each article stands on its own, takes ten to fifteen minutes, and has something to operate. The chapters in system design and AI engineering link here when they use one of these ideas without stopping to explain it.