It can be ridiculously over complicated, so I'd get an agent. You do not pay differently because you use an agent, they are paid by the insurance companies, so use them. They can figure out the best plan in your situation. Some don't cover drugs because not everyone is taking a lot of medications, so it makes sense for that person to have a different plan than one who does need the prescription coverage.
My husband's supplemental insurance costs around $400/mo. Seems outrageously expensive to me, but at the same time he had a hospitalization that cost over $100,000. Had he had to pay 20% of that ($20,000) we'd have been hurting, but he paid almost nothing out-of-pocket. My father had cancer. His last nine months cost nearly half a million dollars but his insurance between Medicare and a supplemental policy ended up with him paying almost nothing. Get the supplement. Some cost zero (my mother's is a zero cost and she has an agent that calls once a year to say whether she should stay with that plan.)
So, my advice is find a local agent (if you want someone you can see in person), or one online. They are not going to suggest a policy that doesn't make sense for you, and again, you do not pay differently for using them.
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