This is a nice, Smashing deep-dive by the author of React HereMaps:

Armed with basic knowledge, beginner and intermediate developers of Node.js struggle with many things: “It’s just a runtime!” “It has event loops!” “Node.js is single-threaded like JavaScript!”
While some of these claims are true, we will dig deeper into the Node.js runtime, understanding how it runs JavaScript, seeing whether it actually is single-threaded, and, finally, better understanding the interconnection between its core dependencies, V8 and libuv.