Cadillac is it's own separate entity. Even where it is teamed up with others, they tend to have their own showroom.I think the split after the major rehash of things and the new store design themes was Buick/GMC and Chevrolet/Cadillac pairings, but with a few exceptions as I previously mentioned.
Chevy is it's own pairing, trucks and cars/CUVs).
GMC (trucks) used to be paired with Buick (cars). But GMC has expanded into the CUV category that has mostly replaced cars, and Buick, having dropped it's cars, only exists as CUVs, so that pairing just gets you two matching sets of CUVs in the same dealership.
I believe Buick only exists because the HUGE Chinese market demands Buicks, which they like to think of as American, even though they aren't. If the US dropped Buick, it would be difficult for the Chinese to think of them as American.