At least the Jews seemed to have found a way around it, though - why were they successful? Did they build their own schools, or how did they do it? Afaik at least for some time they were banned from certain professions, which is why many become money lenders and became rich - might have been before the 20th century, though.

I don't know that much about the history of the US, but it seems Jim Crow laws were in effect till 1965? https://en.wikipedia.org/wiki/Jim_Crow_laws

Women, as I said, had other things to do, before the invention of the washing machines.

The question remains, why not found a company hiring those supposedly cheaper workers, and blow the competition out of the water?

I suppose a company hiring only blacks in the early 20th century would have been unable to sell it's wares because of racism?

Are software developers on a "market for lemons"? Because employers presumably can't know how good a developer is, they bid too low, which in turn makes good developers avoid the market altogether? (ref https://en.wikipedia.org/wiki/The_Market_for_Lemons )