You are discovering the difference between HMOs and PPOs. While in a HMO you can only use the doctors and facilities designated by the plan. Prior to joining the plan you need to be sure that all the doctors and medical facilities you use are part of the plan or you will have to select different doctors and facilities. Often HMOs cover more than PPOs, such as dental, eye care, chiropractice and other, also usually cover medical prescriptions with limitations.
PPOs usually do not cover medical prescriptions. You need to get additional insurance to cover medicare part D at an additional cost. However under PPOs you can choose from a wide variety of doctors as long as they are licensed and in a recognized field of the medical profession. The same goes for hospitals and medical facilities. Medicare only covers part (usually 80%) of any medical service. You need to get additional insurance (known as Medigap policy). These policies come in many flavors all described and controlled under Medicare supervision.
There is much more info but I hope this will give you an idea of what you need to study.