I need help to understand who genesis was written by…

I need help to understand who genesis was written by (argument that doesn’t make sense for Moses to have written…how could he have known). Also lost books of the bible and the argument that bible made up of series of human decisions about what can and cant be included.

I’ve been researching but quite new and either stuff is too simplistic and not answering the arguments or far to complicated and I’m not understanding it.