He is a legend in fitness and has been in outstanding shape for his entire life. What I read in the past about his training about 25 years ago is that he used about 3 exercises a body part for 3 sets of 12. He did nothing for his legs except run distance and sprints.
Of course as he aged I'm sure he does things different now. Arnold wanted him to compete but he had no interest. This guys has been in athletic shape decade after decade starting in his teens to now. Just a freak.