Medicare Supplement (Medigap) Plans are sold by private insurance companies, and help pay most (if not all) of the health care costs that Original Medicare doesn't cover, like copayments, coinsurance, and deductibles.
Medicare Supplement Plans are standardized, so the key factor in choosing one is to first determine which of the several plan types best fits your needs and then to simply find a strong company with a competitive premium.
There are no networks with Medigap plans and if a procedure your Doctor feels you need is covered by Medicare then no Prior Authorization is required. When you have Original Medicare and a Medigap policy, your provider sends the bill to Medicare and they pay their share of the Medicare-approved amount for covered health care costs. Then Medicare sends the bill on to your Medigap policy, and they pay their share. If there is any balance owed after that (very often there is none!), then the beneficiary (you) is billed the balance.