How does Nuxt 4 implement edge streaming SSR for sub-100ms TTFB?

/ server/api/checkout.get.ts
export default defineEventHandler(async (event) => {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue(new TextEncoder().encode('<div>Loading...</div>'))
      const data = await $fetch('/api/cart')
      controller.enqueue(new TextEncoder().encode(`Total: $${data.total}`))
      controller.close()
    }
  })
  setResponseHeader(event, 'Content-Type', 'text/html')
  return stream
})