They must have calculated this by last name. I don’t see how they can categorize people as “German.” Ok, I have Germans in my ancestry, but also have Scots-Irish, Scottish, Welsh, English, Bavarian. My grandkids have all that plus Portuguese, Algerian, Serbian and Czech.
I don’t think the European get how mixed we are. My granddaughter’s kindergarten has a large proportion of mixed race kids. That is why we are more “American” as a people than anything else.
I think it’s self-declared, and only the first ethnicity given is used.
If they went by last name, most African-Americans would be listed as English and there would be no AA pluralities.