To answer your lower question since I can't respond directly, the legal difference between a rifle and pistol is pretty much only barrel length. Overall length also matters, and whether or not the AR "started out" (at manufacture) as a rifle or pistol determine if it becomes a "short barreled rifle" after certain modifications, but that's getting into the inane and nonsensical world of ATF regulations.

As far as what an assault rifle is, it's basically a rifle capable of burst fire or automatic fire (it's a bit more complicated than that but that's the basic idea). Rittenhouse's AR (and virtually all civilian owned firearms for that matter) are not capable of burst or automatic fire and are not assault rifles.