What makes it a power grid is that the stations and substations are connected in a mesh topology. Every major load center is going to have multiple possible paths for power flows from multiple generators. In most cities, even residential distribution will have multiple feeder paths.
In most cases these are controlled by switches at the substations that put circuits in and out of service, thus moving the path current must take. More modern options include phase shifting transformers and other reactive device, and even impedance reactors that will adjust flow across multiple live circuits.
Aside from that, changing where generation is happening also moves current to different circuit paths, so for example a "peaker" gas turbine plant that can ramp up quickly might be brought online in one location electrically closer to the load to balance flow away from an overloaded or to-be-disconnect circuit.
In most cases these are controlled by switches at the substations that put circuits in and out of service, thus moving the path current must take. More modern options include phase shifting transformers and other reactive device, and even impedance reactors that will adjust flow across multiple live circuits.
Aside from that, changing where generation is happening also moves current to different circuit paths, so for example a "peaker" gas turbine plant that can ramp up quickly might be brought online in one location electrically closer to the load to balance flow away from an overloaded or to-be-disconnect circuit.