Managing multiple app environment domains

Question

I am building a web app with a couple of friends (as a hobby).
We were wondering how to deploy the application and manage 2 environments:

1. For future customers
2. For us, the developers, and other business staff

I was thinking at first creating 2 domains: app.example and app-demo.example,
but I don't want customers to find the app-demo.example app.

This led to to wonder, how does big companies manage their environments DNS?
How can Facebook limit the access for the different deployed versions?

And what are the guidelines to choose a pattern to manage the different live environments?