Can I go to any doctor/hospital, or am I limited to specific medical practitioners?

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.