I think it would be difficult to recount your whole life chronologically right off the bat. But I think that the writers and the subjects of the books engage in long and many conversations over a period of time - memories and context are bound to add up as you converse and dig deeper. Also, I’d presume that interviews are done with other people who were either involved in specific events, time periods, or knew the subject of the biography well, further adding context and richness with details and perspectives. But these are all guesses, I don’t really know

As you mention, I think it depends on the type of fiction book. For example, if it’s a book written on a subject like economics or history, I would rate the book based on the integrity of the research (if I’m able to) and how well the author(s) manage to distill complex topics to a wider audience and build a narrative.