USA gymnastics, also known as USAG, is top-ranked country in the world in gymnastics. Other countries include Russia, Romania and China. But as for the last decade, the United States has been on top, winning numerous team awards and medals along with many first place all-around Olympic gold medals for the last couple of Olympics in a row.
USA gymnastics is more scientific. There is a set standard of what each level should be progressing at in order to pass to the next one. Start at level one, at your basic gymnastics, and go all the way up to level ten, and then something after that called elite gymnastics, which would be your Olympic or collegiate level.
Each level has been written out, it has videos on how to do that level in order to progress to the next level. But ultimately United States has scientifically come up with a winning strategy in order to successfully compete in the world of gymnastics and be number one for the last decade.