Computer Fundamentals: No BS

I've been working with software for a few years now, and I've noticed something uncomfortable. I can build things. I can work with React, Node.js, databases, APIs, cloud services, and all the usual stuff that comes with being a software engineer. But if I stop and ask: "What is the computer actually doing underneath all of this?" My mental model gets surprisingly fuzzy. I know the concepts. I've used them. I've probably explained some of them before. But knowing how to use something and understanding what is actually happening underneath it are two very different things. And I want to fix that. Why I'm writing this This isn't a course, and I'm not writing this as an expert teaching computer science. These are essentially my notes while rebuilding my computer fundamentals from the ground up . I'm trying to connect the things I use every day as a software engineer with what is actually happening inside the machine. Instead of learning concepts because they're on a traditional CS syllabus, I'm starting with a question: What do I actually need to understand to reason about a production system? For me, that means being able to look at a system and understand what's happening underneath my code. Why is something slow? Where is the bottleneck? What happens when something fails? Why does adding more memory help in one situation but not another? What actually happens when two things execute concurrently? Why does a database query become slow? What happens to a simple HTTP request between two machines? I don't want to just know the answer. I want the mental model that lets me reason about the answer . The path I'm taking I'm roughly following the layers that a typical request passes through: CPU → Memory → OS → Network → Storage → Concurrency → Distributed Systems So the series will go through: 1. What Actually Happens When Code Runs Starting from the bottom: CPU mental model, memory hierarchy, and number representation . 2. Operating Systems Then moving up into processes, th