Factory labor (Labour)
From memory:   It was a London clockmaker who is credited with mass-production-line techniques. However, the workers were time-served clockmakers and trade apprentices.
When factories became the norm in the industrial revolution, the owners provided housing for the workers that they needed. These were not at that time 'slums,' but houses for respectable families who would not otherwise have been able to be there, and workers flocked from the farms and workshops to live in them. they were 'tied' houses, and if you lost your job, out you went.
These houses may have become slums later, with factory closures and private ownership, but no-one 'invented' them.


If we consider semi-skilled workers, then possibly the first mass-production was of Springfield rifles, so America can take a bow for that.
This is from memory of a book I read once. I stand to be corrected.