If your only income after retirement is SS, then it is not taxed. If you have other income to go with it, like a 401k, IRA, or such. Then all income is taxed.
SS takes care of Medicare and Medicaid also. In the 1960's SS had a huge surplus, thus adding Medicare and Medicaid out of SS. Then they started trying to balance the budget using SS tax income. That means they borrowed from it to pay for their pet projects. Now SS is running out of money.