They sell the same networks in my experience (west and northeast coast).

The MCOs are not carving out different networks for specific employers. You buy a bronze PPO from United Health directly, or your employer does, it’s going to be all the same providers. If you get gold, then the deductibles/copay will be less.

The networks used to be different when HMOs were in vogue, but if your employer is only giving you an HMO option, you need to find a different employer. PPOs are the only sensible option (except maybe Kaiser on the west coast, but even they sell PPOs, just costs more to see a non Kaiser provider).

This was not my experience on the NY exchange but that was several years ago at this point. So I guess it's possible that things improved.