With React by itself, you need to know the API's URL. You should read about environment variables and how React handles them.

You would have one environmental file for local development, and then in production, you'd use Render.com's environment variable settings.

One of these variables would be something like REACT_APP_API_BASE_URL (doesn't matter what you call it, as long as it begins with REACT_APP).

Then you prepend all of your API calls with that in your React code.

Next.js does not have this setup step because it serves your API and static site from the same s Node server by default, so all of your API URLs would just be /my-api-call (because the API and fronted would have the same hostname, whatever it is).