It's Medicare PART A, PART B, PART C, and PART D. These are US Federal Government grograms, even though C and D are contracted out to private insurance companies.
Supplemental Insurance Plans: Plan G or Plan N etc. are provided by private insurance companies. If you choose PART A and B, you can buy Plan G/N to cover that 20% of PART B "Approved Amount."
medicareschool.com's Mr. Musick explains well. He has videos on YouTube, among others, who are insurance agents.
