Torah from heaven

Hello,
I read the trilogy and learn that the rabbi makes a clear distinction between a verse/obligation from the Torah that we are obligated to even without full understanding, and the laws/authority of the sages that must be understood and accepted if it is correct.
My question is – what does the rabbi think about the writing of the Torah? Was it all given to Moses from heaven as it is? Or was it written retrospectively after many years (as the research suggests)?
If it was written years later, how can we distinguish between a verse spoken by the hero and guest writing/commentary by the editor/writer?
 
Thank you very much.