Dotenv runs on the server. If a part of your application also runs on the server (such as with Next, Nuxt, etc.), and your responses are generated dynamically, then it is trivial to use the dotenv. If, however, the runtime of your application is confined entirely to the browser, and what you are serving to the browser is only static assets, such as with create-react-app, then, if you still want your environment variables to be defined at runtime rather than at build time, you need to design a bridge between your server and your client specifically for injecting the environment variables.

Here, for example, is an article describing such an injection at the nginx level: https://medium.com/@jans.tuomi/how-to-use-environment-variab.... Or you can serve index.html using a full-fledged dynamic server, such as Node, and inject environment variables there (and probably cache the response for good measure as well, not to overstrain Node). This really is not something that is React’s responsibility — although it may very well be the responsibility of the frontend developer to set this all up.