Content creators on Youtube have a bootstrap problem. Once their videos become popular enough, Youtube's content discovery mechanism will direct potentially interested viewers to said videos. The problem is that if no one has seen the video, then the algorithm has no idea what its quality is, or what type of viewer would be interested. Depending on your target audience, even getting a small number of views by piggybacking a popular channel may be enough to get you onto the algorithms radar, which generates a lot more views that the click through rate would suggest.
Linking within the video works, but it requires that the creator of the original video does more work, and if it was not planned when the video was created then, while doable with annotations, it appears scammy.