Enrollment starts 10/15 to 12/7. This is only for the Part D drug plans and the Medicare Advantage health plans. These have no underwriting. (Traditional Medicare supplemental Medigap plans have enrollment any month - and you may be subject to underwriting).
The very popular drug plan Humana Walmart has split into different plans. This link is to the 2020 comparison chart. What will be important is the forumulary (the drugs a plan covers and what cost tier they are in). The Basic plan does not restrict preferred pharmacies to Walmart and Sam's Club.
You can view all 3 choices, along with other plans, at the Medicare.gov website search for drug plans, with your list of drugs and choice of pharmacies. This will give you the plan list sorted by estimated annual cost to help you choose.
Medicare's website to compare plans: