Question about WPMUDEV structure

Hi,

I have a blog platform i want to develop on a new domain installation.

but I want to understand why WPMY domain is a subdomain not a domain like: wpmudev.org?

why premium.wpmudev?

2. I saw that users subscribes in to that subdomain page and non on the domain page (wpmudev).

How is possible please? How may i let my user join my multisite on a subdomain?

Thanks