>>717119349
They do typically go into their new series with a planned ending that they default to if their series gets axed. If their manga ends up being popular their editors tell them to not end the story and instead extend it.
Take Dragon Ball for example. The series was originally supposed to end with the defeat of Emperor Pilaf. The next planned ending was with the defeat of Frieza, but Toriyama's editor convinced him to continue it. Then Toriyama planned to end it with Cell, but was convinced to continue it again. Finally he managed to end the original run of Dragon Ball with Buu.