Moses is the author my Jewish tradition and by the authentication by Christ Himself.
However, it is understood that (1) at least one portion of Scripture had to be written by someone else: the latter part of Deuteronomy— after Moses’ death; and (2) even if Moses had an amanuensis, it doesn’t strike his overall authorship. Very similar to Paul: he didn’t actually write most of his letters, and he states so.
Moses is the supposed author of the Pentateuch. He couldn’t have written the portions of the Old Testament that are set in the time periods following his death.