shoe benders have a roller forcing the tube into a curve on the inside..

Muffler shops use that type equipment.. you can see the Imprint of the shoe .
mandrel benders add a solid plug that will keep the tube round as it's bent.
it is held inside the tube as it is bent.

wall of tube thins as it is stretched, around curve, outside , thickens on the inside..

the really good tooling redistributes the metal so wall thickness remains consistent ..