I'm pretty sure they search the title, tags, and comments too. And since the Google algorithm also works on what words in link text embeds and links can cause a video to get noticed as well. Hence why some channels don't allow comments or embeds and use all those weird hard-to-input characters in the titles and such.
Mirroring works quite well to get around video blocks when intros don't work, and for song uploads people usually have to pitch-shift things a tad.