The subway runs through that square underground, and then a few above ground lines connect to it - there is no need to go underground anywhere except that small downtown area. You can walk across the entire downtown area from north to south in about 15 minutes.
The corresponding city in the U.K. would be something like Newcastle.
The greater SF area is much more than the the area called San Francisco. I mean I might as well tell you that the City of London is a tiny square a mile on three side with a few tens of thousands of residents.
"SF" is not used to refer to the "greater SF area", which is rather "the bay" or "SF bay area", a reference to the San Francisco bay which all these areas adjoin.
Think of all the cities along a river. What they have in common is the river. They are not all called by the name of just one of the cities. The SF Bay is 4000 km^2 and there are hundreds of cities all along that bay.
The east bay has berkeley, and Oakland, the south bay has Palo Alto and the cities along the Peninsula and then Santa Clara county with San Jose (which has more people than SF). The North bay has Marin. These are all independent regions with their own history, economy, and demographics, and identity. They do not call themselves, nor are they commonly referred to, as "SF", nor does the SF muni service these areas, they have their own separate muni and lightrail. BART (bay area rapid transit) is the regional lightrail throughout the bay. Economically, SF is not even the most dominant of the regions along the bay -- Santa Clara county is much richer and home to more businesses and more people.
Back to your point, just because you are aggregating across a very big region, adding up all the people that live in that region, does not mean you can meaningfully compare that 4000km^2 area with a city and then complain about the differences in urban amenities between an actual city and the large geographic region.
The various cities along the bay are quite small - only San Jose cracks a million -- and none of them are dense. It is almost entirely single family homes with backyards, interspersed with very large parks and nature preserves.
That's like saying "London has, what, 50 million people"?
We have left the realm of plausible errors and are far into "no idea what is talked about" territory.
SF is a 7 mile x 7 mile box with mostly flat, single family homes. It's ~850,000 people.
Within that box is a small 1 kilometer x 1 kilometer square that has the big buildings.
Here you can see an aerial view of the top half of SF, with the downtown area seen as a little section in the midst of a pool of single family homes:
https://www.thepinnaclelist.com/wp-content/uploads/2017/07/0...
The subway runs through that square underground, and then a few above ground lines connect to it - there is no need to go underground anywhere except that small downtown area. You can walk across the entire downtown area from north to south in about 15 minutes.
The corresponding city in the U.K. would be something like Newcastle.