There is no porting, which is why MS just needs the publisher permission to make the game backwards playable.
They don't touch the existing code at all.
It certainly doesn't appear that way.
[video=youtube;lPQuDyWULNU]https://www.youtube.com/watch?v=lPQuDyWULNU[/video]
It's certainly not a one size fits all solution. They are either shipping a translated binary, or a custom VM for each title, the details are bit sketchy. This thread also has a bit of info: http://www.kotaku.com.au/2015/06/how-xbox-ones-backwards-compatibility-will-work/
One guy mentions his 55MB game became a 800MB download.
I'm very skeptical they can emulate 360 hardware on the xb1 in software, full speed, without doing some optimization. Don't get me wrong, it still technically pretty awesome.