Ignoring speed-of-light delays, it's completely in-sync across the whole grid. Operators are required to keep things as close as possible to 60 Hz at all times,

Ignoring speed-of-light delays, it's completely in-sync across the whole grid. Operators are required to keep things as close as possible to 60 Hz at all times, but it does vary over time. To be clear, frequency is not the only signal. Operators have voltage and current monitors on most transmission lines, so they can tell where the power is coming from and going to. The whole system is partly automatic and partly held together by people making phone calls.