you can do it any way you want.
some people say that you need a relevant domain name for whatever the niche is.
that helps, but is not necessary.
you can register a bunch of domains and just point them at folders on your main website.
you can setup complete different websites each with it's own domain name.
you can create subdomains from your web site control panel and put your niche site in that folder for that subdomain.
( a subdomain is just a folder under your main site that has it's own setup on the server. It will be subdomain.yourdomain.com )
you can do either one of these things, or all , or any combination, on one hosting account .