SSI (Supplemental Security Income) is frequently confused with SSDI (Social Security Disability Income). SSI is welfare and is never taxable. SSDI is treated just like Social Security Retirement Income: i.e. it may become partially taxable only when added to your other income.
People with SSI have no reportable income and do not normally need to file a tax return nor get any benefit from doing so. Social security benefits (including SSDI) only becomes taxable when added to sufficient other income. Most SSDI recipients do not normally need to file a tax return nor get any benefit from doing so.
But, along comes "stimulus" payments, to mess up "normally". If you were not someone else's dependent (or qualified to be someone's dependent), you are entitled to 2020 and 2021 stimulus payments. If you did not get them (you had to use the non-filer tool at IRS.gov), you can still get them, in the form of "recovery rebate" credits, by filing a 2020 return. The 2020 recovery rebate credit ($1200 + 600 = $1800) will be your 2020 refund and the filing of a 2020 return will cause the IRS to send you the $1400 2021 stimulus (if you don't get it, filing a 2021 tax return in early 2022 will get you the 2021 recovery rebate credit).
