There have been changes in the demographic makeup of the country. Census records have shown that minority groups have increased as a percentage of the population. The reasons for this are varied, but don’t involve any Great Replacement that the Republicans think is the case. And while immigration is part of the demographic change - as it has been in the past - it’s not the only reason.
Still, people like Trump see more brown and black people and say “they must be illegal immigrants here to ruin our wonderful white country.”