Not exactly true with MEDICARE. On January 1st, yes, Open Enrollment begins. It ONLY applies if you signed up or have a Medicare Advantage Plan with Drugs (MAPD). You can not switch to one from PFFS (Paid Fee For Service, aka "Basic") Medicare, nor can you switch from one MediGap plan to another without penalty (if you do, you may incur underwriting and higher premiums). But, you can drop from MAPD to Medicare PFFS/MediGap. If you ONLY have Part-A and you want to sign up for Part-B (like if you worked past 65 and now want it), you can sign up then, but the Part-B effective date will be July 1st.
Also, the whole Turning-65 Enrollment Period is all year (3 months before your birth month, your birth month, and 3 months after your birth month), assuming you signed up for BOTH Medicare Part-A and Part-B. So, if your birthday is in April, you can sign up between January 1 and July 31st. Signing up between January 1 and March 31, your effective date is April 1, while signing up on your birth month or later is the following month. If you're turning 65 in 2020, adjust those dates to show when you should sign up. I suggest finding an independent agent, one who represents many carriers, since they can find you the best deal. That is what I do in Florida, I represent several carriers.
There are lots of other enrollment periods, mainly they deal with a change in your life. If you move, your income drops and you qualify for Low-Income Subsidy, enter or discharged from a nursing home, or drop/lose employer coverage. As of 2020, your doctor stops accepting your insurer is not a reason, I'm guessing because finding a participating doctor is as easy as looking on your insurer's website, calling your insurer, or calling your agent.
TS