Whiteness is obviously based on

No, it isn't obvious. "It is obvious" is not an argument. This is social science prevarication. You sound like a UCLA sociologist! Going on about laws and human attitudes--we have the science to assess this empirically. So let's.

Jared Taylor has a video on the history of race that I can find for you, but he did a damn good job at showing how consistent the definition was all throughout the years.

Jared Taylor did a terrible job in that video, which you would not know if you didn't take the time to verify the information for yourself. Don't watch youtube videos to get your information. Read "The Rise and Fall of the Caucasian Race." THAT is the gold standard for comprehensive history of Whiteness. It's a 300+ page book, not a shambolic youtube video put together by a closetted Asian Supremacist like Taylor.

where do physical and social characteristics come from? Why for example, would the U.S pass laws like Jim Crow or use the One drop rule if they didn't believe that Whiteness and Blacks were suppose to mix?

I see this far too often on this sub. A facetious deference to science coupled with a curious refusal to actually speak scientifically. It comes from being uninformed about genetics at a sufficiently high level to speak compendiously about race, and a fear that a leftist who knows more might show up and drop unfamiliar terminology. So we pretend to respect science while engaging in precisely the same silly sociological/anthropological humanities equivocation that leftists do.

Genetics is the ONLY way we can discuss race empirically. Everything else--legal, geographic, social, historical--is just a crude reference back to science. We HAVE the tools to discuss this correctly. Any focus other than genetics is simply inferior.