The partitioning of Poland or its « migration » to the West ? Because as far as I'm aware, the split is due to the Western part being formerly German (including its population) rather than the Polish people itself having been split. Polish people are welcome to correct me, I'm not too familiar with history in this region aside from the broad stokes.

You are referring to the idea that some of these phenomena might not be due to the era of partitioning that ended in 1918 [1] but to the territorial changes after WW2 [2], in which people from former Eastern territories were relocated and mixed with the population of the "Regained Lands" in the West.

Interesting observation. It is hard to tell it apart, because it is the same line but different time.

I am no historian, so I shouldn't even try explaining. But for me, the "partitioning" explanation makes more sense, because during the communist time after WW2 there was a strong trend towards the unification of the country. If not for that, we would see even stronger divides at the other sides of the partition lines.

[1] https://en.wikipedia.org/wiki/Partitions_of_Poland

[2] https://en.wikipedia.org/wiki/Recovered_Territories

Only parts were strictly German. And the German population was moved out to new borders, and Polish population from east (parts of current Ukraine, Belarussia and Latvia)

That's pretty dank alright.