Create files in the pages/api/ directory (Pages Router) or app/api/ directory (App Router):

Pages Router:

// pages/api/users.js
export default function handler(req, res) {
  if (req.method === 'GET') {
    res.status(200).json({ users: [] });
  } else {
    res.setHeader('Allow', ['GET']);
    res.status(405).end(`Method ${req.method} Not Allowed`);
  }
}

App Router:

// app/api/users/route.js
export async function GET() {
  return Response.json({ users: [] });
}

export async function POST(request) {
  const data = await request.json();
  // Process data
  return Response.json({ success: true });
}