How are they subdomaining this WORDPRESS

These plugins might help:
https://wordpress.org/plugins/automatic-subdomains/
https://wordpress.org/plugins/wordpress-subdomains/
https://wordpress.org/plugins/tags/subdomains
http://codecanyon.net/item/category-subdomain-pro/6567807
https://webostock.com/market-item/wordpress-page-as-subdomain-pro/8860/
https://mythemeshop.com/themes/sociallyviral/

What's the point though? Better rankings?

Most of people are use to the wordpress because in the wordpress is very easy in the web 2.0.It is feature of very nice. More people are create the subdomain and install in the wordpress. And more than benifits in the wordpress of the web 2.0.

Wordpress is very easy of the working and good feature and in this wordpress of the more than the social media plugging. As well it will provide many services.

Wordpress provide more than most popular theme and more than pluggins etc. Wordpress is strore the more than data and recover in it. Wordpress provide nice feature so that you can see your nice created website in the market.

The trick here is a wildcard redirect at the domain level. For example, this A record


Once you have entered this in your domain, every sub-domain (no matter if it exists or not) will be redirected to your server IP. Now, you can use your htaccess to get what sub domain is being requested and show content according to that.

http://corz.org is also your friend if you are looking for htaccess tutorials.

Edit: As I see, you are looking for a wordpress solution. Even then, the first part of my answer is valid. As for the site part, install WP multisite.

they are using wordpress multisite plugin