How do you build backend API for Next.js apps?

Do you use the Next.js own router and create routes under the API folder? Do you use a custom server? Or do you create your own backend server, serve the next.js as usual - via the built-in server and use a reverse proxy in front of them?

Of course, the approach would differ based on what we are trying to do. Have you got any preferences, though?

And what approach would you opt for, if you'd want to create an ecommerce in Next.js with a heftier backend?

Let's discuss!