I guess you mean the housing that you see coming on Caltrain after pass through south San Francisco? 30% of SF’s housing is classified as single family, so there must be some in the city. Just not as common as it is say, here in Seattle.
Not quite. The Brisbane project was an old Southern Pacific (Union Pacific?) yard that's right by the Caltrain station. AFAIK there's still nothing there.
South of the Mission take a look at Oceanview/Merced Heights/Ingleside (OMI). That's largely SFH. BVHP (Bayview + Hunters Point) as well and parts of it are visible from Caltrain. Heading west you'll find basically all of the Presidio, Forest Hill, Sunset, and Richmond neighborhoods are SFH or maybe low-rise apartment buildings.
A lot of the single family houses in SF have walls right up next to each other but do not actually share a wall. While Forest Hill properties historically had racially themed covenants, most of those neighborhoods predate HOAs and the worst of the covenants have been discarded.