Next.js API is back-end?

Yes. Next.js is a pre-rendered React app in the client-side that users can view and interact with and can be considered as front-end. At the same time, it also does server-side rendering and API routes which can perform server-side code and access data in the database and can be considered as back-end.

Dynamic Routing has two folders pages or api. To stay on the front-end or client-side put your JavaScript / React DOM code in pages. The back-end or server-side of Next.js is with the api folder. The api JavaScript code is not executed in the browser but with Node.js so this is not safe for static page generation. When Next.js is hosted in the cloud with Vercel then your server-side JS will render, but on Netlify or other static hosts the api folder will be ignored or throw errors. It's like running Express.js code in the browser the JIT rendered will fail

Next.js api provides REST API. We are sending requests internally to our next.api routes. With this you can add business logic in your next.js project without writing any additional custom server code and without configuring any api routes. In node.js app, we need to separete api code into controllers and routes, then register each route in express app, then you need to make sure that you registered the routes in correct order.

The only drawback as of now, vercel does not support websocket connections. So you cannot have realtime services. But you can kinda make it almost realtime with SWR. More on SWR Basically, with swr, you tell next.js to fetch data periodically, keep the response in cache and serve it.

With this feature, Next.js provides everything to build a full-stack application. So next.js also simplifies the backend for us.

Your Question: "Then i want to know "Next.js api is back-end ?""

Simple Answer: No

For any technology to be backend it must fulfil its minimum criteria to identify itself as backend.

NEXT JS is based on Serverless Functions and is not a backend

You can't use NextJS like ExpressJS.

A better word for NextJS API would be Middleware.

I come from the asp.net world and am slowly, enjoying, learning javascript development. It does clash with me though when people describe something like Next.js, which I'm enjoying learning, as a backend.

To me it is more conceptually like an asp.net server-side frontend (Webforms, MVC, Razor Pages) etc. It seems its main concern is producing reactive frontend UI. Just because some of the frontend functionality runs/processed on server does NOT make it a backend AT ALL.

What about the fact it has api endpoints?

Well to me, a web api is only a means of enabling a frontend to talk to a backend across a network, the internet in this case. For me an api handles only very limited concerns; provides the endpoints, handles de/serialisation, talks http to the caller and that's kind of it, thin and dumb.

The backend doesn't really start till you get to your App layer and then the architectural fun can start, Application orchestration, Domain logic, dependency inverted data access layer etc. etc. all the things that talk to the complexity of designing robust software.

So much talk seems to be focused on where bits sit, is it on the client, is it on the server and less about the concern of what goes where.

I thing Next.js is an excellent server-side, frontend framework but whatever your onward stack into the backend, it's not a backend!