Conclusion
You will receive an invoice from Medicare for your Part B premium and Part A (if you lack premium-free Part A) If you are not getting Social Security or RRB benefits. Part D IRMAA charges may also be added to the bill in some cases. These costs are paid in advance for the coming month or months.
References
[1] https://www.investopedia.com/terms/m/medicare-part-b-premiums.asp
[2] https://www.hhs.gov/answers/medicare-and-medicaid/what-is-medicare-part-c/index.html
[3] https://www.medicare.gov/drug-coverage-part-d/how-to-get-prescription-drug-coverage
[4] https://www.cms.gov/Medicare/Health-Plans/Medigap
[5] https://www.healthline.com/health/medicare/is-medicare-part-a-free
[6] https://www.ssa.gov/policy/docs/ssb/v68n2/v68n2p41.html