If geography and population density have nothing to do with it, then I suppose California isn't one of the world epicenters right now.

Australia has a lower population than California, by the way. Obviously, its pop is mostly spread over the east coast, but compared to the UK it's extremely sparse, with no tunnels connecting it to the mainland. It's disingenuous (at best) to think that the island nations that had successful lockdowns were all doing something magically better than other places with significant restrictions to daily life like France, Italy, California, etc. because they couldn't realistically close their borders to ground traffic.