Wow you just asked questions that would require about 10,000,000,000 answers

Lets start here

1. Video from the man himself Mr. Matt Cutts - Matt Cutts (Works for Google)
2. Great Web 2.0 Page create from Google themself - (Google Them self)
3. Older but still relevant description about how "backlinks" affect PR - (Google Them self)

Stay away from article directories IMO.

That is a question that no one here can answer. We cant speak for how Google does things internally.

but.... as a web / database programmer for 14+ years let me tell you how its "generally" done

Usually when you have to link to separate sets of data together (ie. database or tables) there is usually a unique_id created to link them which usually is never changed. So when a new record is created that record will live with that ID for its life, also known as a (unique identifier which tends to be an auto-incremented number that is dynamically generated and can not be repeated).

Since records tend to be linked this way, any other fields that exist in the record (firstName, lastName, Url, blah blah) then can be changed without the original ID being disturbed.

So to answer your question from my experience I would assume Google links from a unique identifier of some sort and not the URL directly.

Hope I didn't lose you, its my favorite subject...but no one here speaks that language to much

Now that was a good answer