You don't have to sign in to use You Tube. You just type in the name of the video you want and if someone has put it on You Tube, it will be there. For example, last year we watched the entire BBC series The Great War on You Tube. I just looked up the order of the episodes on Wikipedia.
When we are studying a certain topic, say for examle, the water cycle, I just go into NeoK12, or Brainpop, or Top documentaties to see if there is something on the water cycle.