How a website knows if a video has been watched or downloaded?

there’s a video lecture streaming in a university platform that runs javascript, Can the owner detect if i’m downloading the video (via IDM) rather than just watching it ? In other words, How a website knows if a video has been watched or downloaded?

After much trouble such as quality fluctuations, unexpected changes in playback speed and frozen screens (also experienced by other users), I’ve decided to download the videos using (Internet Download Manager). That, however, violates the terms of the university, even though I obviously don’t intend to make any commercial use of the videos and I’m going to be the only one watching them. I’d like to know if the content provider can notice, that I’m not streaming the videos but downloading them.