Bourbon is an American whiskey, distilled from at least 51% corn and aged in new, charred oak containers. Though Kentucky is known as the spiritual home of Bourbon, the spirit can be made in any US state.

Bourbon is made with a mash bill, aka the “recipe” of grains in the mix. That mash bill must contain at least 51% corn, but the other 49% can be any other grain: wheat, rye, oats, quinoa, you name it. It can even be 100% corn, if that’s what the distiller wants.

Those grains, “the mash,” are then cooked with water, and yeast is added to ferment the sugars. After fermentation is completed, which usually takes about three days, the liquid is now considered “distiller’s beer.”

Distillation is the next step. Producers can opt for column stills, which produces a lighter-style spirit, or pot stills, which produces a richer, more robust spirit. Either way,

Bourbon must be distilled at no higher than 160 proof, or 80% alcohol by volume (abv). After distillation, the clear liquid is called “new make spirit,” although the more colorful term often used is “white dog.”

Most distillers then “proof” the whiskey by adding water, because of a second regulation: Bourbon must not be higher than 125 proof (62.5% abv) when put into the barrel.

https://www.wineenthusiast.com/culture/spirits/the-ultimate-guide-to-bourbon-from-barrel-to-bottle/