NOT KNOWN FACTS ABOUT SSI CHECKS

My partner & I'll both be turning 70 in 2024, so We are going to start out getting social security. I understand that 85% are going to be taxable & that we are able to specify around 22% be withheld for federal tax. So how Is that this finished? Is barely eighty five% on the month to month check taxed at 22% or is the whole monthly amount taxed? I