How do the announcers know almost instantly why a flag is thrown?

Are the announcers REALLY that good at noticing the tiniest of penalties; such as an illegal block?

or does someone tell them through the headset. Even then, how do those people know so quickly?

I understand some penalties are much more obvious like pass interference or offsides.

I just feel like the officials are the only ones watching close enough that would know the penalties instantly.