This is mere speculation because, as you rightfully said, writing on Medium is just a black box - but I strongly believe that the curation occurs with by an automated process. I.e., I think there is some A.I. powered mechanism that classifies stories as curated or not in some topic according to some criteria, like keyword count, which publication was it published in, etc. Since machine learning algorithms get tuned, the more they are used, it would explain why stories published in some publications or by some authors are almost certainly getting curated.

Definitely there are some human editors who also interfere in the process, but I believe they work as an additional step in the process, for example to remove certain stories from distribution if they violate Medium rules, as it happened in the past.

I don't believe it would be possible to have a team who would manually approve or not every published story for publication.

And in terms of topics vs tags, they are simply not the same thing. The tags are defined by the author and the topics are assigned in the distribution process independently.