In Do '..-backports' repositories automatically take precedence over the standard repos? in sources.list, the answer says that backports are never automatically used to update a standard package in the newer versions of Ubuntu.
How is this actually implemented? I've read something about pinning, but there is no /etc/apt/preferences file on my machine, and yet it shows the behavior explained in the answer.
Where is the special status of the backports repository actually configured, and how does apt know which packages have already been updated from there?