How are all scientific publications uniquely identified ?
Formally, they aren't: There is no system to uniquely identify publications. But, author(s), year of publication, and title are typically sufficient to uniquely identify publications, because authors typically publish different works with different titles and it is unlikely that distinct author(s) with the same name(s) will publish in the same year with the same title. Moreover, it is even more unlikely once publication venue is added and distinct once page numbers are added (assuming no two publication venues share the same name in the same year). So, a full citation should suffice to uniquely identify publications, but there may exist (with low probability) publications that cannot be uniquely identified.
(Uniquely identifying authors of a publication is more problematic.)