Glad you have figured out Medicare.
Do you know how Medicare breaks Medigap plans down to make it easier to compare plans?
https://www.medicare.gov/health-drug-plans/medigap/basics/compare-plan-benefits
I got my Medigap policy back in 2009 so my plan is no longer offered. It also covers the deductible for Part B. I got grandfathered in. I never see a bill as long as I go to any doctor that takes Medicare. And I have been to a lot of doctor's these days!!!
And I could self-refer to Mayo Clinic!

Then you want to understand Part D plans!!! Very important to compare your drug costs each year on the Medicare site (there is a great tool for this). It is amazing the difference in drug costs from one plan to another. Check carefully. In 2021 I didn't check carefully and ended up paying $900 more than I had to!! All because I enrolled in a plan that counted Tier 1 and 2 drugs against the deductible when almost all plans don't. So I had to pay retail price until $480 deductible and then copay still more expensive than other plans. And no one can explain to me what extra benefits one gets with the more expensive plan compared to the cheaper plan!!! I have asked!