How do mangaka draw the white lines in a 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.