How are website makers made?

[Fin]

I've always wondered, how are the websites that allow you to make/publish websites made?
Like Wix, Webs, Yola, Moonfruit, 1&1, 123-reg etc
Surely if these websites have the ability to register domains, anybody can do it?
Say, if i want to create a website that allows people to also make websites and register a domain name, how would I do that?

Thanks