What is specific to Missouri
Medicare rules are federal, but which plans are actually available to you is decided county by county. That is the first thing worth checking.
What is the same everywhere
The enrollment calendar is federal, so it does not change when you cross a state line. Your is the seven months around your 65th birthday. The runs October 15 through December 7 every year. A opens only when a qualifying life event — such as losing employer coverage or moving out of a plan’s service area — gives you one.
Late enrollment penalties are federal too. Going without builds a penalty that is permanent once it starts, and delaying without active employer coverage does the same thing to your monthly premium for life.
What actually varies
Which plans exist for you is decided at the county level, not the state level. Two towns thirty minutes apart in Missouri can have meaningfully different sets of options, different networks, and different drug lists. That is why the first question I ask is your ZIP code, and the second is your list of doctors and prescriptions.
I do not publish plan lists on this site. They change every contract year, and a stale list is worse than no list. If you want to know what is available where you live, ask me and I will pull it up with you — or check every option yourself at medicare.gov.