So before this I only worked with simple 8 bit Atmel MCUs and I realized on my development board schematics it has only a 12Mhz crystal, yet the MCU operates at  up to 100MHz. (I think the default is 80MHz. I only made it higher once for fun. It's just a simple line in the code.)

How does it do that? Why does an Atmega328, for example, run at the used crystal speed?