Not sure what method Ancestry used. I believe they got their initial input from the Sorensen Molecular Genealogy Foundation and then probably used other sources as well. One of the most sophisticated admixture analyses is on 23&Me, which has an excellent explanation of the method used. I can't post urls here but if you go to the 23 and Me web site and then search for "ancestry composition" you'll see how they established their reference populations.
PS--this has nothing to do with US immigration; they only included people from "non-colonial" nationalities, although that of course depends on what time period is being considered.