I normally come across the pages I edit in my everyday research. These often include articles that were last edited a long time ago, stubs, articles without sources and other obscure articles. I also used Wikipedia to extensively study aviation from 2012 to 2014 but did not edit during this period.
I upload files to Wikimedia Commons. Most of these have been linguistic vector image maps I've created, usually where no map existed before. I've also uploaded my own photos of notable landmarks and third-party free-use medical images under CC-BY or CC-BY-SA licences, and sometimes I've vectorised existing maps, cropped photos or otherwise improved existing files.