Mozart died before he completed the Requiem. His widow Constanze persuaded his friends and colleagues to assist with its completion. The most likely person who made completions was Sussmayr, who was a family friend, and perhaps (???) had spoken with Mozart about his intentions for the rest of the work. I don't think it's known how much is Mozart, and how much is Sussmayr. But there are various differing opinions about what should be the "final" edition of the work.