If an index file is missing in a folder, apache uses its own file/files to list the contents of the folder.

Since I don’t want to create an index file in every folder in the file systems, I wonder where I can find the file/files that apache uses when there is no index file.

It's been a day so what I did was pop your title question into google. I think it's a wee bit more complicated than one file to change and as such may be best to research this with your title question on a google search.