I've never heard the term staging segment. What you need is a staging environment.
Staging is its own environment, similar to development and production. It runs its own instance of the app with its own database, storage, and everything.
The purpose of staging is to manually test changes in an environment that very closely matches production before you deploy them to production. You don't usually run automated tests in staging, as those are for development and CI environments. However, it should have everything you have in production, such as third-party API integrations.
In the case of a regular web app, the staging site would have its own domain or domains. You might run it on a separate server, in a Docker container (if you also use it in production), or even on a production server. Do whatever makes sense in your case.
For example, how and if to hide this from search engines and similar.
You can block external access in many ways. The easiest way is to add Basic authentication at the web server level. That way you'll be prompted for a password the first time you access the site. You could also use IP whitelisting, firewalls, VPNs, etc. But I wouldn't overthink it.
