I’ve downloaded part (I guess) of the bylaws of the Democratic Party and the Republican Party in California. I haven’t read them all. But they look similar
At state level, bylaws must be similar everywhere, according this description of the Wikipedia of the state of things in Arizona.
At county level, an example of a County Central Committee bylaws (that of the San Diego Democratic Party) is this; a document (also from the San Diego Democratic Party) about policies and procedures, this.
Additional procedures, it seems, can be found here (for both parties).
At national level, the main document of the Democratic Party, or at least a very important one, seems to be this chart.
Even at first sight, it’s clear that the party, the "people" (what I’ve called in others posts "the Assemblies", using an usual term) is conceived as an electoral, a "candidates supporter", machine, not a decision making group, and that the rules seem somewhat outdated.
Of course (as in ancient Rome) there’s always another question: How many parties do you really have, when you have two parties?