How could someone write a biography about someone?

Was sitting on the couch pondering the question of how people get so much information to write about a person? I mean were talking literally about someone's entire life almost day after day?

Eh.....writing is all about literary tool.....descriptions......first hand accounts.....narrations.....easy to get a few hundred pages together if you're a little creative and interested enough in the topic....

I would guess the author was inspired by the person they write about.