These tutorials will help you to integrate your app with an API backend running on another port,
fetch() to access it.
Ruby on Rails
API Platform (PHP and Symfony)
API Platform is a framework designed to build API-driven projects. It allows creating hypermedia and GraphQL APIs in minutes. It is shipped with an official Progressive Web App generator as well as a dynamic administration interface, both built for Create React App. Check out this tutorial.
C# (ASP.NET Core)
ASP.NET Core has a React project template that uses Create React App. Check out their documentation.