What is it about smoking that causes lung cancer? What happens in the lungs? Please remember, IANAMP (medical professional), so answer in terms that are somewhat understandable.
I wanted to be a doctor when I was a kid, but never quite made it.
Ditto me on the medical professional bit, but I believe that since the lungs absorb large amounts (over time, of course) of so-called “carcinogens” (being chemicals that can cause weirdness in cells, namely mutation and unchecked growth), your cells eventually react negatively to these chemicals. The fact that the carcinogens hit the surface of the lungs is what causes it to be localized there.
On which chemicals specifically cause the cancer, I couldn’t say. I would guess there isn’t any single culprit, but that many different ones gradually take their toll.