Its based on what you search for on your computer. We now know what you search for in your spare time.
Not all the time. It does use fillers as well.
Some of the stuff that has popped up, I have never searched for. Besides that, I have the settings set to not remember anything (as far as cookies, cache, localStorage, IndexedDB goes).
However, one does have extensions such as noscript, adblock plus etc. Noscript is more aggressive (will need to set it up), but because it blocks JS, it does castrate a lot of functionality as well. Really hurts sites that use JS more then they should.