Ask HN: Any good resources for learning back-end tech as a front-end engineer?

I've been a developer for a while, but many of the little details of what's happening behind the API remain a mystery to me. Does anyone have any favorite resources specifically aimed at helping engineers with front-end experience gain a deeper understanding of what's happening on the other side of the stack? I'm particularly interested what's going on closer to the hardware and the more arcane aspects of network infrastructure.