US car license plates

In Ontario, car license plates stay with the owner when a car is sold, how is it in US? Do these stay with the car or with the owner?

you'd have to check with the DMV for whichever state you're going to. They're all different AFAIK.

Yes, it depends on the State, some require you to get new plates every time, some let you keep them, some let you keep them up to a certain date (e.g. if they issue new ones), some let you keep them but you have to re-use them within a certain amount of time otherwise they become invalid.