The “.com” and “.org” and all other Top Level Domains are owned/controlled by some organization.

Com and org are your original TLDs, so since they were around first you see them everywhere. At some point countries got their own TLDs so Mali got “ml” for example but Tuvalu got “tv”. (Yes, technically “.tv” has nothing to do with television.) And a few years back there was open bidding for a bunch of new TLDs which is where “.sport” or “.dentist” come from.

Anyone some entity owns/controls them and then can sell any word or domain under it. So if you want “greatgatsby.com” you have to talk to the “.com” owners. If you want “greatgatsby.sport” you talk to the “.sport” owners. Usually there is another company or agreement that groups these together so you can manage all your domains in one place.

So anyways now you own a domain like “greatgatsby.sport”, what do you want to host? Mail at “mail.greatgatsby.sport”? A website at world wide web aka “www.greatgatsby.sport”? Up to you.

Over time, largely by convention “www” became where you put your website.

From there you have two options, you can setup a redirect from “greatgatsby.sport” to “www.greatgatsby.sport” or you can do a little hosting “trick” and just make “greatgatsby.sport” return your website.