to the layman download means the whole video is stored in its entirety on my disk. and possibly to lawyers too. that is not happening with streaming videos. youtube videos in particular most likely never hit the disk. not even the cache. when i am watching youtube then i am not deleting the video after it is done. the "downloaded" segments disappear automatically as i watch. so there is no delete step that i could skip in order to keep the file. rather, actually downloading a video requires an entirely different operation.

Technically watching a video only streams video chunks into memory. But of course there are tools pretending to be a youtube client to stream it to a file.