For environment variables just create a file with the name .env and add the firebase config to it in the format
API_KEY= your_api_key
AUTH_DOMAIN= your auth domain
PROJECT_ID= your project id
STORAGE_BUCKET=your storage bucket
MESSAGING_SENDER_ID=your message sender id
APP_ID=your app id
MEASUREMENT_ID=your measurement id
Then restart the server by running the command
expo start