There is only one paper (no separate letters), which can have multiple authors, if the finding was the result of collaborative work. How the collaboration is do

There is only one paper (no separate letters), which can have multiple authors, if the finding was the result of collaborative work. How the collaboration is done and what authorship implies varies a lot by field though. In mathematics, the expectation is that all people listed as authors of a paper contributed something significant to it and have co-written (or at least read and endorsed) the overall paper, so it really is a jointly authored work (if that's not true in a given case, there will usually be a footnote explaining). In some areas of the natural sciences, though, there can be large numbers of authors on a paper, because a study is the result of a lot of people's work. But in that case there isn't necessarily an expectation that every single author has verified and endorsed the entire study. For example some authors might be students who contributed a specific experiment, so got their name added as an author on the paper for their contribution to the study. But they may not be knowledgeable enough to have evaluated the rest of the paper, and probably also shouldn't be blamed if someone further down the analysis chain manipulated the data. A convention in parts of the natural sciences with large author lists is that the junior scientist (postdoc, PhD student, etc.) who did the most practical work is listed as the first author (and called 'first author' or 'lead author'), while the most senior scientist, responsible for overseeing the entire project, is listed last and is the 'corresponding author'. Those two then get the most credit and responsibility, while 'everyone else' is sandwiched in the middle between them. A 'corresponding author' is, literally speaking, just the author designated as being in charge of correspondence about the paper, with the journal editor and/or with readers who want to follow up (in some journals they're the one whose contact info is printed, although in other journals, all authors' contact info is included). In my field, computer science, 'corresponding author' tends to be mostly a bureaucratic role, the person responsible for handling emails from the journal, and is usually the same person as the first author. In other fields, it can carry more of an implication of the person in charge of the study. In some, the senior scientist's name is first, so they'll end up being both 'first' and 'corresponding' author. Really no consistency across areas, so the terms only mean much when talking to someone from the same field. For example, this paper has 15 'authors' (same as 'co-authors'), of which the last one listed is the 'corresponding author' (indicated by the little mail icon): https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1574-6968.2000.tb09242.x https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1574-6968....