It's heavily team-dependent. I was on a team at Google where I was the only American citizen, with teammates from Iceland, England, Vietnam, India (4), Taiwan (2), and China (2). I was also on a team where there were basically zero non-citizens, and most of my teammates were from places like Ohio, Indiana, Tennessee, Alabama, Colorado, Connecticut, Massachusetts, etc.
Teams do self-segregate. What often happens is that the team manager ends up recruiting people they are comfortable with, which in practice means recruiting people who are culturally similar. If the team grows, the same dynamic works on the next level of managers, and so on. So immigrant managers tend to recruit other immigrants, native-born managers tend to recruit other native-born Americans, and so on. The manager of the first team was an Icelandic immigrant; the manager of the second was an American-born person of Indian descent.
A similar dynamic works on sex, as well. About half of the teams I worked at were roughly 40% women; the other half of them had zero women. Women are generally reluctant to join a team where they will be the only woman, and also word gets around about who the female-friendly managers are. So those female-friendly managers get teams that are close to 50% female, while other managers get teams of zero women. (Statistically, Google Engineering was about 10% female and IIRC 50-60% foreign-born, which means that I encountered about twice as many women that I would've been predicted to and about half as many foreigners. I wonder what that says about my personality.)
