The 20th century belonged to the United States for basically six reasons, as best as I can figure.
1.) Permissive immigration laws from founding through mid-century that made the United States an attractive and attainable destination for anybody seeking a different lifestyle for practically any reason.
2.) During the 1800s the United States granted patents and indemnity for anybody who stole industrial knowledge from another country. Stealing diagrams and machine tools from Britain and smuggling them into the United States was an amazing way to jump up a couple of class levels, especially coming from late 1800s Europe which was the most economically unequal period in known history.
3.) The mass militarization of the United States during the Civil War. After the Civil War ended, the United States was left with the most powerful, well-armed, and well-trained military in the world. This has been true ever since. That's made the United States a very important ally for any country interested in surviving a World War.
4.) The United States suffered relatively light casualties during the World Wars.
During WW1, the United States lost 0.13% of its total population. Canada lost nearly 1%, the UK lost more than 2%, Germany and France lost more than 4%.
During WW2, the United States lost 0.32% of its total population. Canada lost 0.38%, the UK lost 0.94%, France lost 1.44%, and Germany lost more than 8%.
Most of those deaths were soldiers, i.e. working-age male conscripts. The working age male population varies from country to country, but the rule of thumb is about 25% of the total. So multiply by 4 to understand the economic impact. In the run-up to the US gilded age, Canada had a 4% per capita work force disadvantage against the United States.
5.) European productive capital was devastated by both total warfare and capital expropriation during decolonization. Basically, Europe didn't have enough manufacturing capacity to satisfy their own needs. American industry was more than untouched by the war, it was actually bolstered in order to meet the military's needs. Europe needed to buy its goods from somewhere and the United States was the only country capable of providing.
6.) The demand for US manufactured goods created urgent need to establish an international monetary system. Because there was so much demand for US goods anyway, I'm sure it seemed like a good idea at the time to have countries settle their accounts in USD even if the US was not involved in the transaction.
In other words, it made the USD the world's reserve currency.
This created a lot of demand for USD. That means the value of USD went up. That means imported goods are incredibly cheap in the United States.