Prior to Obama, low wage workers might be scheduled to work full time without health insurance. If they're poor enough, they might get Medicaid from the state; otherwise they get to ruin their credit and claim bankruptcy if they show up at the emergency room.
Obama passed ACA, which required employers with full time workers to provide health insurance or pay a fine per worker. Full time is defined as 30 hours per week or more by ACA. So in low wage jobs, employers responded by scheduling people for 29 hours.
So if you work multiple jobs or a job+doordash/uber/etc... you may work full time, and not have any health insurance because no one employer is going over 29 hours.
ACA also expanded Medicaid, but some states decided to decline that expansion. So whether that covers a low wage worker depends on where they are, the size of their household, and their total household earnings.
For many of these workers, it's not just the risk of unexpected medical bills that they can't pay... they rarely have savings, so if they miss a few days of work (say because they contracted COVID), that can spiral into a series of missed bills. And with the eviction moratorium coming to an end, that now includes losing their home.