Actually c % 16 means the remainder of the integer division of c by 16, not the quotient. This comes from the language C (or it could be even older, since C is successor to B, which was a simplified version of BCPL) and is used in C++, Java, C# and others.
Yes, basically each bit in the video RAM corresponds to 1 pixel in the display.
Most machines use more bits per pixel so that can support colors or at least shades of gray (or green, or orange, depending on the monitor). The original Macintosh also used black & white display with 1 bit per pixel, so we're in a good company there.