Reference Document
https://www.robinwieruch.de/complete-firebase-authentication-react-tutorial/
This is React Project /src folder files
You should put your firebase connnection info to your React Project root folder, for example .env file (linux) like:
REACT_APP_API_KEY=
REACT_APP_AUTH_DOMAIN=
REACT_APP_DATABASE_URL=
REACT_APP_PROJECT_ID=
REACT_APP_STORAGE_BUCKET=
REACT_APP_MESSAGING_SENDER_ID=
REACT_APP_APP_ID=
REACT_APP_MEASUREMENT_ID=
Enjoy your tutorial!