Nor me. But Brexit is reliably misrepresented in the US news media, which I also don't understand.
For myself, there's tremendous value in the notion of countries, borders and loyal citizens, but also of sympathy for refugees to the extent a country can absorb law-abiding newcomers, and also of international treaties which make trade and travel as frictionless as possible.
That isn't a nationalist position. I daresay many people would think it sounds sensible. It means a person believes in the importance of both sovereignty and cooperation, of the interests of citizens and of refugees. It's silly to treat these ideas as alternatives, when we want a blend which values everyone's interests as far as possible, while recognising that interests sometimes conflict and in life people have to make hard choices. But ultimately, it's wise for a country to decide its own fate. That isn't the job of some consistently imperialist next-door-neighbours, or of allies across the ocean.
If Germany wishes to add 1m immigrants in a single year, so be it. But it cannot become an obligation for the UK. If Merkel thinks it is, then we have to assert the need for borders. And of course folks think, maybe the idea of the EU isn't so good after all if the German Chancellor thinks she is making decisions for the EU without anyone else agreeing it. Now she wants a European army. Everything she is doing points towards an imperial Europe. Same old idea. Same old cause of friction.
I don't oppose immigration to the UK at acceptable levels. But a mass wave of immigration simply isn't viable. I didn't oppose a low or no tariff Europe but I have no interest in a European empire.
The US must also decide what it wants to do with respect to immigrants. Maybe it can continue to absorb vast numbers of them. Maybe not. I guess the recent election suggested many folks are concerned by the volume. I haven't studied hard enough to know if the recent volume is unsustainable or causes more costs than benefits. Before there was an America, the original colonial idea was that North America could absorb an almost unlimited number of new people and they would be a net benefit to the New World. But that idea can run out of territory.