There are several guides and tutorials to help you study MERN stack. But they only help you to learn syntax and some other basic knowledges, not the real problem-solving, or infrastructure. This guide is for the developers who wanna be seniors. This helps you not only how to code like seniors, but how to write reusable, scalable and well-tested codes.
There are thousand ways to build their own infra. But working 9+ years with MERN stack, I've came with the idea of great infrastructure how to handle requests & responses, errors & messages and so on. This guide helps you to learn that infrastructure. Once you build it, you don't need to worry about those things. This handles all of the things you should have to take care, automatically.
This sample guide is written in typescript, that can help you learn easily. If you are not familiar with typescript, don't worry. Read it as javascript, only without the type.