It is not, other than sometimes in the case of equal contribution. The first and sometimes second authors are the most important, and the last author is often the advisor/senior researcher supervising the work.
This is not accurate; it depends on the subfield. As a rule, the more theoretical the subfield, the more likely that alphabetical order is used. See e.g. papers from a theoretical conference like STOC vs. a systems conference like HotOS.