3B Infrastructure is a notebook of plain-English explanations and hand-drawn diagrams on how software is built. Sections cover system architecture (26 topics), database concepts (9), AWS scaling (13), DevOps (19) and a 15-step VPS deploy guide, plus engineering subjects and a hand-picked course path. The maker says topics come with demos you can play and step through. The site says it is free to read with nothing to sign up for.