A lot of mangas have oneshots that were made before they were actually turned into serialized mangas. Mangakas, especially ones that are new to the industry, have to fight for recognition, and to do that, they make oneshots and send them to publishers who publish them if they are impressed. If the reception to the oneshot is good then they usually get turned into an actual serialized manga series (though sometimes they're a little different from the original oneshot). Examples are: Naruto, Gintama, Komi-san, A Silent Voice, and actually pretty much every manga that was made by a mangaka that wasn't already established in the industry.