Nobody knows except Arnold and a few who were close to him and would never share it.
I've heard things ranging from "Arnold used to just grab a handful of dbol and swallow the whole bunch"
to
"Arnold used to pin 3ccs of Primo every single day"
to
"Arnold was a pioneer and used cadaver GH extensively...Nubret was experimenting with monkey GH at the time also"
to
"Arnold was on everything the guys today are on minus high amounts of GH and Insulin. This includes Tren, EQ, Deca, Primo, Masteron, Anavar, Dbol, Anadrol, and a little test"
Nobody will ever know. What we do know is that he trained like a LUNATIC. I think that the "golden age" physiques came down to the level of training. Were they overtraining? Maybe...but they had the best and most aspiring physiques any man could ever dream of possesing.. You don't see bodybuilders look like that anymore.