My mother never wore shoes walking around her Brooklyn neighborhood growing up and as a result even as an adult her feet were bulletproof . She could walk on glass.
Over time, bare feet will develop calluses (just like manual laborers will develop them in their hands). I have the same kind of feet as your mother as I can comfortably walk on most surfaces barefoot, including snow. Eventually my feet do get cold walking in snow so I don’t do that for very long!
If your mother had done that in the South in the 1950s, she mostly likely would have also had hookworm.