The US Grid is split into three relatively unconnected grids, east coast, west coast, and texas. Each section has one plant designated to keep the frequency, sort of a "master plant" that is used to balance instant load. All of these are synchronized across the US using phaser measurement units that are timed to GPS.
The other cool thing about AC power is that you can send power based on the relative phase of a node on the grid. This means that a node at a lower AC voltage can send power to another node at a higher voltage by adjusting the relative phase. Mostly this is done by switching capacitor banks at various transmission nodes to adjust the reactive power component at that node.