Many people are confused by the terms Social Security and SSI---they are not the same. Do you get Social Security benefits or do you get SSI?
WHAT IS THE DIFFERENCE BETWEEN SOCIAL SECURITY BENEFITS AND SSI?
https://www.ssa.gov/ssi/text-over-ussi.htm
For the stimulus check:
If you get Social Security then each year you receive a SSA1099 from Social Security in late January/early February showing your Social Security and the amount you paid for Medicare.
Social Security and Stimulus Payments
https://home.treasury.gov/news/press-releases/sm967
If you receive SSI--you do not receive any sort of document to use on a tax return and do not file a tax return that reports the SSI. Here is where to register for the stimulus check.
IRS PORTAL FOR NON-FILERS
https://www.irs.gov/coronavirus/non-filers-enter-payment-info-here