Earlier quoted context omitted.
Next.js does server-rendering and has API routes built-in. No need for Express or Fastify unless you like the DX of those frameworks better.
The Parent parent was talking about if you don’t use Next and want a more sophisticated setup then then those are alternatives. The parent then asked why do you need to integrate [react] with Express and my answer was if you want to do server side (not using Next)
Using Express and Next together is relatively common (or at least used to be): https://github.com/shdnx/next-express