Medicare is like any other insurance company — there are monthly premiums into an insurance pool. I work in insurance, and I can tell you right now that your Oscar Healths, Aetnas, and United Healthcares all represent the monthly cost to administer some insurance for some pool by that monthly premium. That's THE WHOLE POINT of insurance, that everyone pays the same and the output is unevenly distributed. It's shared actuarial risk.

Medicare likewise operates the same way, except it's government-run and non-profit. The government could, in theory, run the exact same insurance service, and collect monthly premiums to pay for it. The UBI could include in it the amount necessary to afford the Medicare monthly premium for everybody.