How to set up test environment web development

Hi I am learning Django and very new to web development and programming in general.

I have seen people using 2-3 environments/subdomain I am not sure what it is called.

For example if website is www.mydomain.com, one would be using www.qa.mydomin.com or www.test.mydomain.com.

are thse envoirnment/subdomain are mapped to server with single ip address or are they different servers. Is it possible to map them to single server.

It'd be great if someone could explain this in context of Django and digital ocean droplet server.