.env files are a popular language-agnostic solution to configuration and secrets management. At first glance, they are extremely simple, which accounts for much of their allure.
Just make a file named .env (or .staging.env or .production.env or .whatever.env) and fill it up with KEY=VAL environment variable pairs for any configuration your app needs for a particular environment. You (hopefully) put this file in your .gitignore so it doesn't get committed to version control.
When your app starts, it slurps up the file via a 'dotenv' library for whatever language you're using, and then you can access your config as environment variables.