How do video authors find snippets to insert

[j7n]

Video authors on the web often insert short clips from a feature film or a television programme, for example, the StarTrek guy facepalming, or a businessman saying to another that "plastics is the future" or the line saying "a few moments later" in a strange accent. This also sometimes happens on Facebook where a gif is added to the comments section. Sometimes the same clip is reused, but often we see new ones never before picked by this author. How do they find these? Surely they can't watch a whole film to find them while also producing their video in a timely fashion. Does there exist a database of these that they search?