This will vary for different insurance plans. Some plans allow you to visit any medical practitioners, while others have their provider network.
In the latter case, if you visit a doctor/hospital within the provider network, the fee will be a standard rate that has been agreed between the insurance company and the provider.
However, if you visit a provider outside of the insurance companies provider network, there may be a difference between the amount charged to you and the amount the insurance company considers reasonable. In this event, you will have to pay the difference between the two.