How is insulin produced in genetic engineering?

They take one cell that produces a protein, for example, e. coli, that lives in the gut of humans and they remove the DNA

Then they take the actual DNA from the insulin producing gland cells, and they insert it into the empty nucleus of the cell. Now you have a cell which has a "computer program" telling it to produce insulin instead of the original protein it was meant to produce. The cell can reproduce and produce more insulin. Then they actually "harvest" human insulin. In general, this is the same way that they duplicate human growth hormone and other human or animal hormones.