the more i learn about history the more i am convinced that the modern concept of a monocultural nation-state (that is, a nation-state that is primarily distinguished by being of "one people") is an inhumane invention, mostly constructed to aid & operationalize colonialism.
people just didn't live in monocultural worlds for most of human history and it's bizarre to desire that, unless your goal is to better define cultural heirarchies to expedite/justify resource extraction from the "worse" nations. which, of course, happens all the time in the modern world, so like, i get it. but it's not good and shouldn't be desired by any good person
when usually this actually reflects the fact that their country is dominated by a single ethnic/cultural group to the extent that anyone in that group is seen as inherently foreign, and therefore not a "part" of their country that they could be racist against, or is seen as being an ethnic/nationalist group and not as a "race" per se.
obviously the US, Canada, etc. do have problems with racism that are fairly apparent because of how quite a bit of the population isn't white people. but, for example, if anyone from a 99% white european country tries to tell you they aren't racist at all, ask them how they feel about muslim immigrants or about the romani people and you'll get quite a different story.
at that point you should maybe stop talking to that person
In the past there was not ethnicities define by countries or countries defined by ethnicities. There was countries full of people of different religious and ethnic groups and there was the ruling monarch and that ruling monarch was of some sort of ethnicity or religious group which may or may not have been the majority and the borders of their domain had nothing to do with the territories in which you could find the various ethnic and religious groups you could find within this particular territory.