An autobiography differs from a biography because it is written by

An autobiography is written by the person about themselves. For example if you wrote a story about yourself it would be an autobiography.

A book written by someone, about someone else is a biography. For example, If I wrote a book about your life.

"An autobiography differs from a biography because it is written by" - the person the book is about. / the person themself

Autobiography is a story about rhe author and biography is a story about another persons life.