I’m no expert, but I might be able to help. Generally and historically in the U.S. one electric company (often called power companies) would handle the entire operation for a local area. They own the generators, long distance transmission lines, and the “last mile” connection to consumers, whether residential, commercial, or industrial. These are expensive systems to build so it doesn’t really make sense to have more than one company providing this service to an area. The power companies are granted a monopoly for whatever area they serve by the state, but in return they are also closely regulated by the state for the business side of the operation, such as how much they can charge for electricity but also making sure they have enough generation capacity and that the distribution system is adequately maintained. They are also regulated by the federal government to make sure they meet technical standards for compatibility. The regulation ensures the companies make a profit by try to keep it relatively small since electricity is a basic necessity. Power company stocks are generally considered a stable, reliable investment. They won’t necessarily grow much, but they should always make a profit.
Each power company is responsible for their own area and the network they operate, often called a grid. However, for resilience they’re usually connected to neighboring grids so that they can get extra electricity if needed. This isn’t free; they will have to buy the electricity from the neighboring grid but it will probably be at a discounted rate. They might also sell electricity to a neighboring grid, of course. I don’t know why they aren’t all just connected to each other in one big grid, though.