You become a "trauma surgeon" by being a general surgeon who takes trauma patients. There is no board-certification in it. The fellowships in trauma are 1 or 2 years, with the two year programs allowing you to get eligibility to sit for your critical care board. These fellowships are for the most part non-operative types of setups (they assume you learned that during your residency) with a supervisory role for the residents in a junior faculty role as well as a heavy emphasis on bench research for priming your academic career (almost everyone who does these fellowships is planning on going into academics as they're not much use for private practice careers)
Many surgeons will do a trauma/critical care fellowship to become chair/director of trauma at a level 2 and sometime level 1 center in a private practice setting. This allows them to be the trauma lord (and paid accordingly) while also continue their private practice. A nice supplement. But really, there's no reason a general surgeon couldn't do the same thing. You just have better odds of getting that type of setup if you've had the fellowship.