dunno if tx has the same system, but i believe (not 100% sure) that ca or at least parts of ca have it regulated so that multiple companies can use the same lines to service retail customers. like there's one line to a house (obviously) and the line belongs to a company, but to foster competition, supposedly to the benefit of the retail customer, there might be a dozen energy providers that could use that line to sell you energy (paying a small fee to the owner of the line).
and yeah, obviously the electricity is all the same so it's just like, the companies dump a certain amount into the grid, and presumably they can bill retail customers for some of that, if they have them, and if they don't they have to take wholesale prices.