I have heard the following line in the TV series "How to Sell Drugs Online (Fast)":
Polizeiarbeit ist eigentlich wie Angeln. Bei dem einen beißen sie und bei anderen eben nicht. (= Police work is like fishing, actually. They bite for one guy but for another guy they don't.)
I understand the metaphor between police/fishers and criminals/fish (referred as "sie"), but I do not understand the article "dem" before "einen". Would "bei einem beißen sie und bei anderen nicht" be wrong or mean something else? Or is "dem einen... anderen" an idiomatic expression (as the illogical/idiomatic expression "on the one hand...on the other hand" in English) ?