Here is the actual state of affairs.
Medicare Part A (hospital insurance) has a trust fund and a payroll tax.
For many years, the receipts from payroll taxes exceeded the benefits paid out. So, the trust fund was in surplus. The extra funds were spent on other government programs and special government bonds were created, but not the kind of bond that is bought and sold on Wall Street. They were basically IOUs the government wrote to itself.
During the initial outbreaks of Covid there was a shortage of ventilators, used to help patients breathe:
I know a doctor who relocated to a small town after being recruited to join a new practice. He sold his house, bought a new one and uprooted his family for a move 150 miles away. It turned out that it was not a lucrative move. His schedule was quickly filled with Medicare patients, most of whom required 30-minute visits due to multiple chronic conditions. He remarked that his pediatrician colleague could see two or three privately insured patients during the time it took him to see one (lower paying) Medicare patient. His income fell far below expectations and he decided to get out.