There are two answers to that question. There always have been.
One response revolves around politics. If you accept the principles of limited constitutional government, you agree to follow the law, you don’t oppress your neighbor, and you don’t expect too many handouts, you can be an American. That’s all it takes.
The second response is predominantly cultural. The most important component is the use of English, but you have to listen to the right music, go to an acceptable church, watch the right sports, and so on. And, of course, there is a racial component, too. Anyone who doesn’t meet these standards, even if they were born here, isn’t a real American.
The second group is currently running the country on all but economic issues. Our job is to bring back the first crowd.
Have a happy 4th!