Hello @hnl66 , I'm happy to help.

This is a multi-faceted question, I will attempt to answer your question fully.

Withdrawing money from a 401k can affect the premiums paid for Medicare part B.

The following link to the Social Security Administration provides more information on how this is determined:

https://www.ssa.gov/benefits/medicare/medicare-premiums.html 

Distributions from retirement plans, such as a 401k are not considered earnings. However, 401k distributions increase the overall income, which can affect the amount of Social Security that is taxable.

When the only income is from Social Security, it is not taxed.

However, depending on the total income, up to 85% of benefits can be taxed.

The following TurboTax article provides information that you may find helpful in determining the taxable amount of your benefits:

https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/taxable-amount-socia... 

I hope that this fully answers your question, please reach out if you need anything more on this topic.