YouTube search is almost completely useless, I just use Google to find YouTube videos. It does seem to search in the video transcript and the comments (I'm not sure but it seems that way).
YouTube's search results will show you like 10 results of the thing you actually searched for, then resort to a section of completely random video suggestions unrelated to your search in hopes some thumbnail will draw your attention and suck you back in to wasting your time watching videos and being served ads.
I also suffered from this issue for a while but then discovered "Unhook" browser extension, which in addition to many other amazing features (e.g. hide recommended videos/comments) has a feature to hide irrelevant search results. With the feature on I get an endless list of matching results and no junk.
