Those are exported raster images of vector art. I use Inkscape, a freeware vector program as well as Paint.net. I use a filter to help but nearly every line is still drawn by hand (actually mouse). Depending on the detail, these can be very tedious and time consuming.
For example, I just did one for a friend. I timed it and it took 18 total hours of digital drawing.
This is a tutorial from a very talented artist who doesn't use any filters: rotoscope tutorial