The US, broadly speaking, does not make a distinction between step-over and step-through two wheeled motor vehicles in any category.

Again, broadly speaking, over 30mph is a motorcycle, under 30mph is a moped (unless it's an ebike with pedal assist, but only in some states). It's complicated.

You used to see a lot of scooters (step-through motorcycles or mopeds) in California college towns but ebikes have decimated that market.