I'm not american but my american girlfriend says people here flirt just for banter, is that true?

Some may. Some people may flirt just for fun or banter, without intending to go further. Others are checking for responses, perhaps reaffirming their attractiveness. Still others may be wanting to incite jealousy in their partners or see if other partners are available. If your girlfriend is the one flirting, you'd need to talk with her in order to understand what she means by her actions.
Subjective. There is not data. Just someone's observation. What about the french? The italians? Anyone? Just an individual opinion. Better not to stereotype!