We could only see Nixon's back but not his face.
why was Nixon’s face never shown in the movie?
The conversations you are hearing are actual recordings from the listening devices which were installed in the White House by/for Nixon.
Using an actor to play Nixon to speak those words would have been redundant. It's much more impactful and dramatic to use the real person's actual voice and film it as though we are just seeing the Oval Office from outside.