Well, then you probably don't need it :) But anyway you can read something about it on Wikipedia.
The usage of it was fairly low and there are better alternatives these days anyway. You can use Virtru or ProtonMail; or many others.
Absolutely not. The email can be intercepted on the way to your alias. And there are other constraints. But your Xmail provider will not be able to read your emails, that's for sure.
Yes, it is built on Ubuntu, Postfix, Dovecot, GnuPG, Apache, Bootstrap, MySQL, gpgit and many others.