People are conflating culture, ethnicity and citizenship.
In Japan, say, an immigrant is unlikely ever to be accepted as Japanese. It is part of the culture.
In Britain, we accept immigrants who are assimilated into the culture as British, no matter where they are from or what their ethnicity is. It is part of the culture.
It is possible for someone who has never been to a country to claim nationality, and even possible - famously in the case of the USA - for a country to claim a foreign national as one of its citizens and to impose income tax on him or her.