You don't have to go fully headless to integrate React into your site. You can use it for a page, a section, a content type, or a page component.
First I'll talk about what React is good for and when it may be a good fit or not.
I'll walk through how we used it on https://nowanthen.com using Drupal 8 and JsonAPI module.
I'll get you started with adding a custom Drupal 8 module to start connecting Drupal and React. And show the ins and outs of the JsonAPI module so you don't get stuck being seen with the core REST module.