Websites like amazon.com, newegg.com, bestbuy.com, etc, have pages,
replete with information, for each and every single product they
sell. My question is... where do they get this information from?

Websites like facebook.com and digg.com have API's that let you pull
information in an easy to parse XML format that, in theory, should
never change, even whilst the layout of the website does. Is that
what websites like bestbuy.com does? Does bestbuy.com pull it's
information in an easy-to-parse from it's vendor?

Or maybe bestbuy.com just uses a bunch of regular expressions and
parses it's vendors webpage that way? If so, they're liable to have
to recode their parsing tools every time their vendor gets a facelift.

The third possibility that occurs to me is that maybe they just have
people enter this information into a computer, all day?

Any ideas? If they have an easy to parse API format, is that
something that other vendors are likely to have, as well?