You can no longer purchase it directly on Steam, though existing owners can still download it. Physical copies or keys from third-party resellers are the only remaining official ways to obtain it. Translation & Files

The .rar file you mentioned likely contains either a pirated copy of the game or, more commonly, an for the original Japanese retail version.

Are you trying to from that archive, or were you looking for a technical breakdown of how its bullet patterns work? Gun Bullet Children | EXceed Wiki | Fandom

The story follows Sowel Devosiana, a girl who joins the "Gun Bullet Children" program to hunt vampires and avenge her parents.

As of January 6, 2023, the eXceed series was delisted from Steam because Nyu Media shut down and was unable to transfer the publishing rights back to the original developer.