Given there are many approaches, and each of them have their own minutia - your question is a little vague to answer easily...
Are you referring to server side rendering?
Or do you mean adding React to a pre-existing Express application with server rendered view engine like jade/pug/handlebars?
Or do you mean connecting React to a Express API?
For the ladder there are many examples:
Regardless, there is no "defacto-standard." There are many implementations. Check out Next.js, check out this.