I can't make any sense of the ordering of comments in topics. Is it by shares? If so, how are tied 7+ comments sorted? It's not by time, since I've seen a comment three hours old at the top, 1 day old one below it and then 16ish hours old at the bottom.
Time (exponential decay) and uphubs, or whatever we call it. A brand new comment will appear up top, and then fall. Uphubs may slow its descent, or push it back up.
The same happens for posts in your feed, although comments contribute to the score of a post as well.