piezoelectric refers to generation of electricity from pressure applied to the material... the inverse of that effect is what generates oscillation.. quartz has a natural resonant frequency determined by its shape, size, and the way it’s cut, and when you apply AC it oscillates at a specific frequency.. the applied electricity causes is the material to deform.. that is the basic physical effect used in oscillators

MEMS oscillators are increasingly replacing quartz in compact, rugged, or integrated designs.

PLL-based frequency synthesis is used to scale a low-frequency reference (e.g., 25 MHz crystal) up to CPU/GPU GHz speeds.