A React app or whatever app developed for web/mobile doesn't fit in Voice "apps" experience. The only thing that (maybe) you can reuse is the same API that the mobile/web app uses to get/store data.
Voice Skills for Alexa or Actions for Google Home are a totally different experience for the user and for the developer.
For an Alexa skill you can complete this tutorial to understand the basics of the development of an Alexa skill. Understand the concepts of utterance, interaction model, etc. Study the code and build your own trivia
Then you can start using the framework from the company I work to build Alexa skills called Voxa. It's really easy to build Alexa skills and the project is in constant active development to keep up with the new Alexa features and soon you'll be able to use Voxa to build Google actions too.
Good luck.