I've never heard that the last author is the primary.

mimipal has more formal academic background than themself.

but I'm not an expert in the finer details of who's who...

it doesn't matter the "finer details," it was more that this seems like it's stating stuff that either, "everyone is wrong about," or it's begging the question?

heard before that the first author is not usually the supervisor but the junior. However, that actually says very little about who really had the ideas and who did most of the work. Each individual case could differ.

Nature's (the journal, not reality) convention is that the principal author goes first, then the 2nd biggest contributor and so on until they're all done, then the person who got the funding goes at the end (e.g. PhD supervisor, grant holder). This is usually how it's done elsewhere, but I've never seen it formally requested by the journal at submission.

sometimes there can be conflicts, e.g. if the grant holder contributes more directly to the analysis than someone else and would therefore appear not last - in which case, they appear last anyway.

some journals have started requesting semi-detailed lists of what each author contributed, in the form of tick-boxes with names like, "coding", "interpretation", "text", "experimental design" and so forth, which is a welcome innovation, in my opinion.