My grandfather wrote 75-80 books and I asked him that very question. He said it depended on the genre. His YA or thriller books, he did not plan the ending but

My grandfather wrote 75-80 books and I asked him that very question. He said it depended on the genre. His YA or thriller books, he did not plan the ending but discovered it along the way. For mystery books, he wrote the ending first and then worked backwards to get to the first chapter. For biographies, he said he'd better know the end or he didn't do his research properly.