Should the first author be the one that worked the most? And then by the amount of work order (descending), but the last one the most-senior researcher?

Are there other ways? Which ones?

Does it vary from field to field? I've heard some areas or even some conferences always ask for alphabetical order. But then there are evaluation committees that count for how many first-author or last-author publications does a researcher have. Can someone provide exhaustive insight into how this is done across many fields?