The NES/famicon has always used the Ricoh 2A03, a modified MOS 6502 processor to generate the memorable music and sound effects of classic Nintendo games and continues to define the newer 8-bit / chiptune music to this day.
Several games included additional chips (known as Memory Management Controllers or MMC) to increase processing power of games. I have always understood that the presence of additional chips can increase memory through methods like ROM bank switching, but I have never understood how certain MMCs like the VRC6, the VRC7 and even the Famicon Disk System were able to add additional sound channels.
Are these MMCs simply memory storage or are they actually just additional CPU's? And how are the additional channels mixed with the 2A03? Do the MMCs route audio through itself and back to the famicon?