Who do I email about a question on a paper?
Papers usually have one (or more) designated corresponding author, or “contact author”, or “author to whom correspondance should be addressed”. That's whom you should contact.

Sometimes it's indicated by a footnote, an asterisk, or at the end of the paper. Sometimes, it's indicated by the fact that only corresponding authors have their email address listed.

If there is no corresponding author, then email either the first author or senior author, depending on the nature of your question and how confident you feel in communicating with them. I would refrain from copying the email to all authors (except maybe if there are only two): if he wants to, the author you will write to can forward your query to others.