How did the 1920 millionaires make their money?

The millionaires of the 1920s primarily made their fortunes through industrial expansion, finance, and the Stock Market. Many profited from the booming automobile industry, real estate development, and consumer goods, as mass production and technological advancements transformed the economy. Additionally, the post-World War I economic boom and speculative investments in the stock market contributed significantly to their wealth. The decade's economic prosperity also saw the rise of magnates in sectors like oil, steel, and entertainment.