What was that surgery called that Trump was victim of?

Scalp reduction is the answer to your question. This is when there is a bald area in the back of the top of the head, and the surgeon removes the bald area and leaves a large scar running down the excision area. This is a permanent scar, and the bald area’s removal usually returns or worsens too quickly and makes the person more bald. Most of the time, in those days, the surgeon chased the recurrent balding with more and more Scalp Reductions.  I know from personal experience because I had three of them done on my head, which left me scarred and with a paper-thin scalp that used to bleed regularly. I then figured out that if I transplanted it, the transplants would restore the infrastructure to the scarred area. I was lucky because I only had crown balding and a lot of donor hair. Still, I suspect that Donald Trump was not so lucky and ended up chasing his hair loss with the old type of plugs, leaving his donor area heavily depleted, as seen in this video.