Next.js how to make a server right??

Question

Express.js

In generalI roughly understand how Next.js works and everything is fine. You need to make an online store. It was decided to write on it. I figured out the front and everything seems to be clear. But as it touched the server, I realized that I don’t understand: D The server is written in express. How to connect all this? as I understand it, next has its own server under the hood, which deals with everything ... I thought to separate the parts and just send requests from the full front server to a separate express API server. But wouldn't this chain be wasting time??? next has custom servers. Express including. But I don’t understand what this means and how to work with it .. Does this mean that it will come in custom to completely configure everything on its own, rather than a next server. An example on the official site next with a custom one on express simply does not start)) Please help me figure it out.