From our own DotGov, it doesn’t look so confusing now.
Choose Original Medicare and you can have any doctor that takes Medicare. Choose Advantage and you can have any doctor in the Advantage provider’s plan.
How much you pay for the coverage depends on all those particulars like co-pays, percentages, etc. and both the MediGap and the Advantage providers (as well as Part D) have plans with multiple levels of coverage.
Six months and counting!