Mostly it's handled like this:
While in development Client (React; handled by webpack-devserver) <--> API (some other server like tomcat)
So the webpack-devserver (or any other node.js server) is serving the static assets of the website. Any APIs you use are hosted somewhere else.
In Production you need a server who serves the website (static) which can be any tomcat or other server or like in some of my cases it's hosted within my service as static content.
Hope this makes any sense :)