Modern backend for frontend on Node.js

The modern frontend is a rich client application operating of a spreading dataset. However, remaining the paradigm of the client-server architecture such applications still require server-side code part of which we, the frontends begin to write, fighting with backends. Along with this come the questions: how to write server code? Can the whole backend be ported to JavaScript? Will it be an enterprise code level? Is Nest what everyone was waiting to start writing a real backend?

Let's figure out what a backend for a frontend is, how to write it correctly, and why Nest doesn't solve anything when we talk about architecture.