[NeoCaveman]

And they always use fresh oak barrels for bourbon that are charred on the inside before filling. The charring caramelizes the sugars in the wood.

That's for bourbons. as you said. Other whiskeys do not char the barrells and hence are not bourbons.

[SoothingDave]

Many of them, and other spirits like sherry, make use of used bourbon barrels.