How does Google know if a page is updated?

Google finds pages mostly by following links from other pages, so that takes care of the 'created' index. I suspect it also looks through updated domain records to find new domains.

I believe updating existing pages is based on the popularity of that page in google search results, though I'm sure they also maintain records of how often a site is updated.

These are just my guesses though, others here may have more info.