They don’t! Depending upon the type of book they want to create authors must research or reimagine the details they choose to recount.
An autobiography bears a greater requirement of factual accuracy for two reasons. First, an autobiography is presented as an historical account of one’s life. Second, an autobiography is generally writen by a person whose life has had some significance politically, socially or culturally. The author is setting forth his or her record of that notable life. His or her reader might reasonably expect that the aquthor his consulted personal journals, calendars and records along with public and private documents pertaining to the events being described. A well written autobiography rests on considerable research to assure its veracity.
A memoir is a different sort of account of a person’s life. A memoir, as the origin of the word rooted in memory suggests, is not so much a factual record as a look backward to search for understanding. It emphasizes feelings over facts. The author asks, retrospectively, what is important among the events of my life? She then focuses upon those events from the standpoint of memory. We all know that our memoir can give events a considerably different patina than the actual events might have had. The memoirist recreates events as she recalls them. Conversations are recreated as she imagines the characters would have spoken. Are all of the details factually correct? Probably not completely. The memoirist is searching for understanding, wisdom and insight born of experience not for the cool accuracy of a newspaper account that passes muster with fact checkers and editors. Memoir is a literary form which aspires to a different type of truth. It is the degree of success the author has in pursuing that literary and emotional truth that is the standard against which her memoir should be judged.