How are Android News Apps built that get updated as soon as new news come in?

How are Android News Apps built that get updated as soon as new news come in?

Almost all of the news sites have there Android Apps which show the news
as soon as they are posted on website. How is this data migrated to APP.
Do they take an RSS/XML feed and parse it or JSON/JSONP.
I am looking to built an APP for a news Site which just gives me RSS feed
and nothing else, I have no control over this website. How do I built an
Android APP for this Site given that I have just an XML feed from them.
The technologies available to me are JavaScript. So I would want to built
this in JavaScript.
I am new to this so please bear with me.
Thanks, Mike