I have a relative who is an Australian citizen. They moved to the US at age 25, became a US permanent resident, worked and paid federal US taxes for ~30 years. At no point did they pay Australian taxes since all income was in US. (I don't know why they didn't pursue citizenship.)
It looks like they are eligible to receive US social security retirement benefits.
If they live in the US until end of life and start receiving US social security retirement benefits does this impact any benefit they might otherwise receive from Australia?
If they decided to move back to Australia and live there until end of life would they be eligible for either Australian or US benefits?