Ideas might come directly from character. A lot of writers start with a character and make that character move forward. What that character wants and what gets in the way of that want is what powers the story.
So let’s say you have a little girl, a tomboy, and she’s got a very distinct Southern voice. She lives in a little town. She has a strange neighbor next door.
Now what? Seems a little generic. If the voice of the girl is strong, though, maybe this will help the writer find her way. Let’s say she has a brother and maybe a friend. They have some adventures.
Things begin to happen but if this story is just about the little girl and her friends, it might be good but it won’t be great. A Southern town during the time of segregation and the Depression and a social structure that creates inequality and promotes prejudice though adds another dimension. The setting is another idea of the author and the development of that setting broadens the story. But now we need some kind of inciting event. A black man is accused of raping a white woman. The girl’s father defends the black man.
I don’t really know how Harper Lee began her story, but somehow many, many ideas bloomed in what could have been the simple story of a girl growing up in a small town. Harper Lee found her way to this larger story situation and then was able to write it so vividly she created a great novel.