No idea why you were down-voted - it's a reasonable question. And San Francisco confuses matters because of the high cost.
In the US, the only time having a true roommate (two beds, one bedroom) as an adult is normal is in a college dormitory.
Beyond that, when American's say "roommate" they really mean "housemate", just as you suspected.
But, rent has risen to quickly in the Bay area, that it is increasingly common for adults to share rooms, not just houses/flats. It's completely abnormal and indicative of serious problems in the housing market in SF. To my knowledge, this is not a common occurrence anywhere else in the US. When it does happen outside SF, it is usually among the extremely poor, which often includes recent immigrants.