Why certain websites are www2 ?

See Wikipedia; it's a short article, but the gist of it is that it's a load-balanceing thing. https://en.wikipedia.org/wiki/WWW2

Within any domain, yoursite.com, you're free to define any sort of higher-level specification(s) that you please. "www." is defined because people today expect it to be there, but there's no particular reason for it. Some domains, like LQ, will (apparently) redirect you to "www." if you simply say "linuxquestions.org," but some do the opposite. There's really no limit to what the owners of the domain may decide to do.

The request comes in, having been routed to your domain by the Internet, and it bears the complete URI-string that the user provided. The server(s) which are driving the domain use this string, among other things, to recognize and to appropriately handle the request. They may or may not "redirect" the client to use a different URI. The details of exactly how the request is handled, and by which server(s), are not visible to the user. Load-balancing may occur whether-or-not the URI is changed, and regardless of what the URI is.