The #1 dictator of vascularity is body fat level, point and period.
Doesnt matter if you have the best "genetic" levels of vascularity, if youre fat, you aint gonna see shit, point and period.
Now, once you are extremely low in body fat, almost ALL AAS will increase RBC's and overall blood volume where it will expand veins much more dramatically than "natural".
Combine that with years and years and years of fattening up your veins from brutal workouts, and the veins literally grow to accommodate all that high pressure blood flow.
As the icing on the cake, you throw in 2 gallons of water per day to intentionally "dehydrate" the body, and you have that crazy ass look.