The four major programs fall nicely into a Matrix: the two columns are the monthly SSA payments (either RIB/DIB or SSI) which trigger the two major medical programs, Medicare and Medicaid.  The two rows indicate which two programs are insurance-based (RIB/DIB and Medicare) and which two are welfare programs with monthly means-testing for income and assets (SSI and Medicaid).

Some individuals get benefits from all four programs, called "Current Benefits" represented by the circle in the center of the Matrix.

We have attached a full explanation of the eligibility requirements for RIB and DIB, which trigger Medicare health insurance, and for SSI which triggers Medicaid eligibility.