Dependent coverage coordination. Unlike group Medicare plans, which may cover dependents (such as a spouse/partner), a standard Medicare plan covers one individual. This complicates the transition from a pre-Medicare group employee or retiree plan to Medicare, since you and your spouse/partner are probably not the same age. work with an insurance agent to set up coverage. AARP article "Younger Spouse Losing Health Insurance, Now What?"
https://www.aarp.org/medicare/faq/qualify-for-medicare-at-age-63/ 

A couple of months before you reach 65 -- and during every Annual Enrollment -- you will start hearing from multiple companies:

Many insurance sales companies will contact you. You will receive mailings and phone calls from health systems, HMOs, insurance companies (such as AARP/UHC) and insurance sales agencies who purchase lists, selected by age, from consumer data brokers. Your postal or email address was not provided by HP/HPE.

Standard open-market Medicare policies are available through insurance sales agencies and directly from health systems and insurance companies. Premiums are based on the highly-competitive open market and across all customers of the insurer. Sales agencies are paid a standard commission and monthly admin fee by the insurance company.

If you have a Medicare subsidy from HP or HPE, you must purchase from a specific nationwide sales agency -- Alight Retiree Health Solutions. You pay the list price for the plan. The HP or HPE Benefits Center reimburses you monthly until your special Retiree Reimbursement Account is empty. ???????
Notes:
- Alight offers most -- but not all -- available open-market plans.
- Some plans are not in the Alight online plan selector -- for example, you must ask for AARP/UHC plans.
- Alight Retiree Health Solutions is a separate company from Alight Benefits Administration, which operates the HP Inc Benefits Center (and the HPE Benefits Center as of 4/1/25.)

Details and member advice: https://www.hpalumni.org/medicare-open-market

You must carefully compare the alternatives yourself. Member advice on how to compare policies: https://www.hpalumni.org/health-compare

No sales agency offers all policies available in your area. Use the official Medicare site to compare benefits and estimated costs -- with contact info -- for all standard Advantage, Medigap, and Drug policies available in your area. https://www.medicare.gov/plan-compare 

Deadlines.  Dec 7 national Medicare deadline to adjust your coverage for the next calendar year. (The national deadline does not apply if you have qualified life event during the year -- such as starting Medicare.)