It depends on the field (and journal, author preferences, and a bunch of other things...), but the main author is generally the person who did the bulk of the research and the last author is the usually the principle investigator, group lead, or gatekeeper (i.e., the person who frames the paper for a particular journal(s) to maximize publication success).

As far as how all of this played out in this case, who knows, but I don't see anything particularly wrong with having either the first author or group lead as the main focus of the article, as long as both of them (and the other contributors) are openly acknowledged, which appears to have happened.

If it matters, the group's Twitter account is worth a read https://twitter.com/miricagroup.

Depends entirely on the field. Different fields have different conventions.

> Except in science the last author of the paper is the most significant one.

In medicine usually. Not all disciplines follow this order.

Actually, the opposite is true

This one is interesting, cause it really depends who you mean by "significant" The first author is generally the primary researcher. The last author is generally the PI of the entire lab, and the person of highest status in the lab.

And in some cases the last author doesn't even read or understand what is in the paper... (Seen that first hand)