A video can only be set to two status:
Monetized, which means that it shows ads
Not monetized, which means it doesn't.
If you upload a video as not monetized, but it contains a monetized clip, the entire video is forced to be monetized. So you could have a 30 minute video and use a 5 second monetized clip, and now the entire 30 minute video is monetized.
However, Nintendo has clips that are forced to not monetized. What that means is that if you use them in a video, you cannot monetize it, even if you want to. So in the above example of a 30 minute video and a 5 second clip, if I want to use that clip I can't monetize the 30 minute vid.
So what happens is that people have 30 minute vids, use 5 second clips, and the rights owner for the 5 second forces the entire video to be not monetized.
However, if you use two contradictory clips - one that forces monetized, and one that forces not monetized, then you can retain your not monetized status.
So, if you want to upload a not monetized video, you can use monetized clips without the video becoming monetized, if you also include not monetized clips in your not monetized video.