I’ve been getting into reading more biographies, and for the most part, it’s been great, but there’s always a lingering thought in my head asking: how much of this is real? I remember large phases of my life, and I get that a biographer would interview people around them/take countless hours trying to remember the past, but how can you remember so many things in such great detail?
Research can take many forms. If the people that they knew are still living then interviews can be a big part of the research. If they aren’t then they look for diaries, letters (to, from , or about), journals, articles written about them at the time, ledgers or any other documentation of their lives.
A good example would be Prairie Fires which is a biography of Laura Ingalls Wilder who wrote Little House on the Prairie. Fortunately she became well known as an author while she was still living so her journals, journals of family and friends, as well as letters written to and from her were preserved. They research also found documentation in the form of land deeds, census and articles so there was a lot of information that allowed the biographer to give a very detailed look into her life.