The US if made off several large reginal grids. The U.S. does not have 1 grid. Each of these regional entities manage power generation facilities (local utilities) and transmission lines.
The U.S. is composed of several regional grids composed of transmission infrastructure and power generation infrastructure. Ipsun Solar is located in PJM territory that manages the grid and we install solar in Virginia, Maryland and Washington DC with Dominion and Pepco as our major power generation utilities. We gave utilities and local Coops a monopoly to manage our power needs to avoid duplicate infrastructure.
Some reginal grids are interconnected, other are not. Texas is notoriously not connected to the other reginal grids.
Consumers in some places can choose between electricity providers. In other places it's a total monopoly and consumers don't have an option.
FERC is the Federal agency in charge of regulating our electric system.