There are physical differences that make some athletes better then others, such as Michael Phelps arms span greater then his body height, but a lot has to do with training. If one thing China has shown, it can find the talent amongst its people to be competitive. East Africans; such as the Kenyans have excelled in marathons. They are genetically gifted but also have lived a life that has conditioned their bodies to use oxygen efficiently and their muscles have developed under those conditions. The US Olympic training facility is in Colorado, a higher altitude location then most other cities in the US. So to just say it’s genetics would be a narrow way to look at it. Taking the genetically gifted and training them their whole life will maximize their potential, at that point it becomes all about will and mental endurance. The US Olympic gymnastics champion from 2016 had a poor showing in the 2021 Tokyo games. She is well trained (as well as an African-American) but her mental state was not in as good a condition and she decided to not compete in as many events, to give her teammates a better chance to win.
It’s not all about genetics.