Get Medicare information from the official source.

Much information about the details of Medicare provided by websites or in the media is oversimplified or out of date -- or influenced by the fact that insurance companies very much want to sell the more profitable Advantage plans instead of Supplemental (Medigap) plans.

Many health insurance websites carry authoritative-sounding information -- and appear to be operated by government, non-profit, educational, or other impartial sources -- but are actually operated by marketing companies collecting leads for high-pressure insurance sales outfits. (Including the official-looking "medicare -dot- com" and -dot- org sites.)  The questionable websites have phone numbers with wording like "Speak with a licensed insurance agent" and "Helping millions of Americans since 1994."

The official Medicare site is http://www.medicare.gov   It is thorough and very well done.

"Coverage and costs vary widely among both Medicare Advantage plans and Part D prescription drug plans. From one year to the next, plans can change their premiums, cost-sharing requirements, and scope of covered drugs and supplemental benefits, as well as provider networks or prior authorization requirements. These changes could lead to unexpected and avoidable costs, and disruptions in care for beneficiaries who stay put and do not at least review their options annually..."

"...Medicare’s official information resources are not widely used... Just under half of beneficiaries reported that they had read all or part of the Medicare & You handbook, and 44 percent said that they (or someone on their behalf) had visited the Medicare.gov website."

--from KFF, the nonprofit healthcare research foundation endowed by heirs of industrialist Henry J. Kaiser. (No connection with Kaiser Permanente. Often criticizes Kaiser's current practices as not being in line with the founder's intentions.)

1. First, check the very clear, official "Medicare & You 2024" book.

Even if you have a paper copy, much easier to use "Medicare & You" if you download the current pdf file. Terms and page numbers are hotlinked to sections in the book. Use Adobe Reader's search feature to find specific words or phrases (Ctrl-F; Command-F on a Mac.)
Download the book here: https://www.medicare.gov/Pubs/pdf/10050-Medicare-and-You.pdf

There are several key tables in the book:

- Pages 10 through 12 explain Original Medicare (+ Medigap + Drug) vs Medicare Advantage plans.

- Pages 13 through 21 explain what you need to do -- and when.

- Page 21 explains how your other insurance works with Medicare.

- Page 76 has table of the Supplemental (Medigap) plans -- A through N.

2. Very helpful articles from the official Medicare site on topics we have discussed on the HPAA Benefits Forum

- Retiree insurance. "If you're retired and have Medicare and group health plan (retiree) coverage from a former employer... "4 things to know about retiree coverage"
https://www.medicare.gov/supplements-other-insurance/retiree-insurance

- How Medicare coordinates with other coverage. COBRA. Coverage based on current employment or current employment of spouse. Workers' compensation. Veteran. TRICARE. And more.
https://www.medicare.gov/supplements-other-insurance/how-medicare-works-with-other-insurance

- What is the cryptic "Notice of Creditable Coverage" letter? 
https://www.medicare.gov/basics/forms-publications-mailings/mailings/costs-and-coverage/notice-of-creditable-coverage

3. If you don't find the answer to your question in the book, use your browser's "search within the site" feature to look up specific topics on the medicare.gov site.

Copy-and-paste this example into your browser, then modify the search terms to suit.
[ guaranteed issue site:https://www.medicare.gov ]
If you search with that example, the official article on Guaranteed Issue Rights comes right up.

4. Check the official Medicare advice and savings site for your state.  State Health Insurance Assistance Program (SHIP) websites have information about local, personalized counseling and assistance to people with Medicare and their families. SHIPs can help you with things like: Your Medicare questions, including your benefits, coverage, premiums, deductibles, and coinsurance. Complaints and appeals. Joining or leaving a Medicare Advantage Plan (like an HMO or PPO), any other Medicare health plan, or Medicare Prescription Drug Plan. Highly recommended by HPAA members.  https://www.shiphelp.org/