Hey everyone,
Socal TRACON is located in San Diego. It used to be, like you said, that the each major airport had its own TRACON underneath the tower, but that has sinced been phased out at most airports. I think that Seattle and Portland may have the TRACON underneath the tower, and possibly Dulles, but the for the most part, the TRACON is seperate form the tower. It also used to be that tower controllers and TRACON controllers were employees of the same facility and rotated between positions (i.e one hour working Ground, next hour working Approach, etc.), but now, with the exception of DFW I believe, the controllers are separate.