Many years ago, Terminal B was heavily invested in to become the defacto international terminal, with a large immigration and customs operation. I think it opened around 1998 or so, and then IIRC, it was refurbished again in the early 2000s. Before that, international arrivals were scattered if I recall correctly. Also, all international carrier flights other than United's and some Star Alliance flights leave from terminal B.

There is a smaller customs and immigration capacity at Terminal C, but B is set up for much higher volume on arrival. All international arrivals, other than some of United's are in Terminal B.

I've landed on UA flights from all over the world, and I guess its capacity and schedule based as to which immigration and customs hall you pass through.