The top-ranking reply is chosen by mods or the system?

Yes, I see that and stand corrected. There have been many changes in the voting system through the years where there were also "Solved" and "Helpful" buttons along with tags of "Apple Recommended" and within the last year were replaced with "Rank". According to AppleJoe, a Community Manager:

Rank of replies is determined by a number of different criteria that we haven't disclosed, but is partially influenced by Upvotes.

You can see a discussion of this change here:

https://discussions.apple.com/thread/256098780?answerId=261476238022&sortBy=newest_first#261476238022

In the link you posted, there are no other users who got more upvotes. That user got 9 votes, are you seeing something else? The next ranking reply is a user that has 7 votes currently, and may change in the future. Something to consider is that just because someone else upvotes a post, does not mean it is correct. Any user who is logged in is able to upvote/downvote a post depending on their own experience and opinion.

Yes. The rank is simply what post has the most upvotes and does not even mean it is the correct answer. It is simply how other users evaluate that post with an upvote/downvote. The mods are responsible for making sure that the forum runs smoothly and users adhere to the Terms of Use, they are not choosing which post is the correct post.

I've also seen that fairly frequently. In general, the post with the most up-votes is also the top ranking reply.

But there are exceptions. Such as, a current top reply may have 3 votes. Then another that had 2 gets a third up-vote. The tie nearly always goes to the reply that just got its third vote. But not always.

Confused? Yeah, me too. There has to be more to the ranking than just votes going on in the background. But only the hosts know what that is.

Sorry, the link refers to location on this site that is only available to users with a higher level and I did not take that into consideration. I think it was OK to post the comment made by AppleJoe for clarification as they are a Community Manager that keeps us updated on changes made with this website.

The link is not available, even I signed in an account for view. It says page not found