Reinvesting Social Security Income

In 2019, being at full retirement age, I started collecting my social security income- mainly because I was tired of writing out a check to medicare. Anyway-

After medicare is taken from my social security, the remainder goes into a checking account. I have not touched the little amount deposited each month.

I heard there was a way to reinvest the social security income back into a retirement account. Is this true? How can I get more information?

 

I think I also read that only a small percentage of social security income can be withheld for taxes....I'd just like to let them hold it all for taxes... instead of depositing it in a checking account (after the medicare payment).... because they will get it all back at tax time anyway.

IS the percentage one can have withheld from the social security income based on the entire amount collected each month...or on the remainder after medicare is taken out?