NCOT Technology has a number of retro-programming videos. Bonus points for using old school DOS Borland C++ to compile the examples.

This gives you an idea of how things were done, but the reality is that demo scene code was a LOT more complex – full of inline assembly, crazy lookup tables, direct framebuffer manipulation, and every programming and hardware trick known to man. You had to be a wizard of not only coding – but expert at tricking the hardware to do what you wanted too.