Mike the link to Killard is in the original link you supplied. Once there the link to Pentatonix turns up. One can go on and on following links which is, I think what they mean by “rabbit hole.”
One of my all time favorite songs. They do an amazing job but I think my all time favorite person was KD Lang doing this song at the Winter Olympics that were held in Canada...can't remember the year.
Consider how significant the aspirin has been to medicine. Whatever ails you, aspirin seems to do the trick. Likewise, when you think of how many artists, how many renditions, how many movies & TV shows where Hallelujah has been reprised, it could be the most compelling piece of music written in our lifetime.