Here’s the priority of the files for the development build and the production build:
Dev.: (npm start): .env.development.local, .env.local, .env.development, .env
Prod.: (npm run build): .env.production.local, .env.local, .env.production, .env
If you ever want to use something in your local environment without being specific to the development build or the production build, you can add some variables to your .env.local file.
Source : https://create-react-app.dev/docs/adding-custom-environment-variables/#what-other-env-files-can-be-used