I saw somewhere that Stalin already knew about Project manhattan before Hiroshima, and that one justification to proceed with it was the worries that “hitter secret weapon” might be a nuke.
The possibility of nuclear weapons was known before the Manhattan Project was created. The Manhattan Project would not have been able to be proposed had it not already seemed like something that could be reasonably accomplished ahead of time.
The idea of nuclear weapons was a common trope by the 1920s, but it was the discovery of nuclear fission by Hahn, Meitner, and their team in late 1938 that made it seem feasible. There were many uncertainties, of course, and that is why no country other than the United States ended up seriously pursuing it.
Stalin knew about the Manhattan Project prior to Hiroshima because there were several Soviet spies working in the Manhattan Project.
Anyway it sounds like you are looking for very basic knowledge about the history of the project. You might find this article of mine useful as a straightforward overview of it.