Talking about retirement. I have always have a doubt about how do retirement pensions work in the usa.

I think 401K (from what I have read here and googling a bit) is some sort private retirements savings pensions. Whilst it is individual choice the employer contributes to it. Ok.... but it is additional to a social security state pension or is it that 401 IS the standard pension that everyone has?

I mean, is there a mandatory minimum state pension or it is the people choice to have one and some people could end with NO pension even if they did work for all their lives?

What is the minimum retirement age when you can start receiving that pension?

My main question is if it is somewhat like in most of euro zone where people are FORCED to contribute to the state pension and therefore have a "guaranteed" pension depending on how much they contributed over their work life or if it is the same as the medical cover where either you pay your insurance for it (or your employer, but not mandatory) or you could end up having to pay for all your medical expenses.