Folks from a given country tend to network with and feel more comfortable with people from said country, affecting their hiring and promotion practices. That’s only natural.
I’m an immigrant and I’ve never felt that way. The U.S. has a melting pot of cultures with everyone able to relate to everyone in some way shape or form. Generally with food. Americans eat German food, Italian food, Indian food, Cantonese food etc. and best of all, we fusion them together…curry pizza for ex.