Ask your employer for a description of theeir supplement plan. You will want to make sure if it is a supplement plan it does 4 things.
1. Pays the inpatient deductible medicare Part A takes if you are hospitalized or pays a portion of it
2. Add more covered days to Part A than Medicare covers
3. Picks up the 20% coinsurance of Part B or at least pays a portion of it
4. Offers you prescription drug coverage (Part D)
When you sign up for Medicare at age 65, you get Part A for free. You are offered Part B but you have to pay for it and the premium is deducted from you social security check. If you work after age 65, you can defer taking Part B if your employer continues to provide employer sponsored health insurance. If you defer taking Part B when retired, for every year you don't purchase Part B, you have to pay a penalty when you do buy it. The same with Part D, if you don't buy it when you can, you pay a penalty for every year you don't buy it. The premise is you have to pay more when you do buy it because you are buying it only when you are sick and need it. Of course as mentioned, for those that work after 65 and continue on their employer sponsored insurance, there is no penalty, but you do have to prove to social security you did have health insurance.
You won't need to take Medicare Advantage from Medicare if you have employer sponsored supplement insurance when you retire because the Medicare Advantage plans try to do the same thing, supplement Medicare but you have to pay for the supplement