There are two stamps that you could get at a POE. One is black and is surrounded by maple leafs (this is Customs - Valid for 6 months). The other is blue and red (this is from an Immigration Officer - valid for however long he or she decides). Immigration Officers verify everything you tell them. We're trained to dig, find the truth, and then admit or deny.
Being American does not make someone special to the Canadian Border Officers. With the exception of NAFTA (which makes it a bit easier to get a work permit), American citizens are treated no differently than any other visa-exempt country.
As for what we are putting into those computers... Usually we're just practicing poetry
