Genetics plays a huge role but there's also a bit of trial and error when it comes to diet/training/compounds. There's no one size fits all, one guy says this worked for him while another guy says something else gave him the best results. Spending time learning how your body responds to training/diet/various compounds will help you figure out what works best for your genetics. Some people respond better to certain compounds while another person doesn't see nearly the same results with the same doses. The top bodybuilders aren't always taking the most gear, their genetics allow them to hyper-respond to amount of gear they are taking. I guess the good news is gene editing technology is advancing rapidly and perhaps in the next 5 to 10 years we'll all be able to edit our genes to make us mass monsters.