Btw. In the last few years german scientist researched the topic of why different ethnicities integrate better or worst in the german society.
It's called...The Bell Curve...
https://en.wikipedia.org/wiki/The_Bell_CurveAnd:
https://en.wikipedia.org/wiki/Mainstream_Science_on_IntelligenceThere's a saying/principle that two people with more than a 20 IQ difference are unable to communicate on any meaningful level. It applies to things like marriage, but in building a civilization as well. If two different groups lived apart and mud huts were the only thing one group could build while the other could build space stations, they aren't going to integrate. In Australia they have Aboriginals which are literally modern day cavemen. Their govt has bent over backwards to try and integrate them into society, but they have the lowest IQ on the planet (something like 60-70), so it always fails miserably.
A bigger factor is that children from third world placed in first world generally excells when given similar opportunities because natives feel sense of entitlement while immigrants relish the chance to participate freely.