How do mangaka draw those white lines which are used to distinguish folds and other indications in the black and white manga?
There are two ways of making white spaces when drawing; it's either by not inking that part, or plainly drawing with white-out. I don't know about the modern manga-kas but the ones working in traditional media use a fountain pen-type to ink. Though the fountain-pen they use normally don't have the ink inside it as the fountain-pens you can buy in normal stores; they have the drawing head and you dip it in ink, or in wite-out if that's the desired effect.
In practice, the white-out is acting like white ink.