Majority of those LEGAL immigrants collect government handouts. They are admitted based on sponsorship by relatives. Among those immigrants are millions of senior parents of legal immigrants, who become eligible to receive social security checks. Keep in mind they never contributed a dime in social security taxes. I personally know such situations.
Elderly immigrants who get legal status (usually a Green Card) are eligible for Supplemental Security Income (SSI) and Medicaid.
SSI is administered by Social Security, but it is paid out from general funds, not from the Social Security Trust Fund.
In other words, SSI is cash welfare for elderly or disabled people who do not have enough work credits to qualify for Social Security or SS Disability benefits.
Here in Washington state, the SSI maximum is $733 per month plus $40 more from the state. I don't know how the max-min SSI payment is calculated, actually.
Also, elderly (65+) legal immigrants who have never worked in the USA don't qualify for Medicare, so they automatically qualify for Medicaid, which, in a lot of cases, is much more costly than their SSI checks.