america is also composed of spanish irish italian black dutch native American german asian culture to name a few

While that may be true it's very much in the same way that Roman culture is the basis and founding of Modern Romantic cultures. It's to degrees and the more derived and hybridized the individual cultures are the more distant they become, not even accounting for cultural drift by time alone.

But I will also note that I don't think the US has a united culture and what is often seen as a Pan American culture is moreso a side effect of the post World Wars migration, the migrations of the great depression/dust bowl, and the yuppies migrating about creating a rough amalgam culture that often out populates the older more divergent regional cultures. Basically if you think of a cultureless suburbanite that's the thing I'm talking about. Also I think mass media reinforced the whole thing.

The foundational European culture of California is Spanish.

While language influences culture and culture influences language they are not the same by any means. Plus within the last hundred years the US has become increasingly mono-lingual in favor of English, according to my great great aunt for example when she was a little girl her father would still speak to himself in a German dialect because German was his first language since he was from a German town. A lot of these minority languages have either naturally due to trade or whatnot or via force from the feds been on the decline or are now extinct some of which predated the founding of the US. A similar thing happened with a lot of regional dialects.

Also define so called high culture? Because frankly given how much of Europe once shared a Greco-Roman high culture irrespective of their own regional cultures I'm gonna call that a potential irrelevancy.