Sometimes literally loud(New Jersey residents raise your hands), but usually also overbearing, painfully extroverted, having to control conversations, injecting themselves into what didn't need them, etc.
I'm not judging from the outside. As an American that at one time was married to an Australian, we met when I was stationed there and we were both at the same base so I would see how my squadmates behaved then later when they had gone off to the racks, I'd get to hear the Aussie service members mock and laugh at some of the behaviors of the guys I came over with. They would joke that I had to be an immigrant because I rarely talked.
It's not an "all Americans do this" type of thing, it's that the Americans that do this are so impactful in a negative way that it becomes a cliched caricature. If you're not that type of person, you won't need to do anything to assimilate, you'll fit in just fine. Those folks were some of the best people I've ever met in my life and they were very welcoming and inclusive.