I think there are sort of 3 main tracks for HPB surgeons out in practice (of course there are probably as many tracks as surgeons but just speaking generally). At very specialized, high-volume, academic centers, like MSK or MD Anderson or a few other places, HPB surgeons are academic surgeons who do only HPB and yeah they do tend to specialize even further, such as just doing livers or just doing pancreas. I think this is a very small % of people who do fellowship training in HPB or who consider themselves HPB surgeons. The middle group is academic surgeons who do maybe 50/50 HPB/general surgery, or some other mix between 25-75% HPB and the other 75-25% in something else, usually gen surg or surg onc or transplant, depending on their pathway. These are academic surgeons, but they tend to do all HPB cases that they can, mostly because the volume just isnt there to be picky. This is the pathway I'm planning on btw.
The other pathway is more private practice HPB surgeon, which again is gonna be mostly a mix of gen surg and HPB, usually starting out as mostly gen surg and building into HPB practice with time and getting your name out there. This also includes going somewhere and starting your own HPB program, whether thats a smaller academic place or a private group. This represents probably half of those completing HPB fellowship training.
I do not think it is common for HPB surgeons to do their own reconstruction aside from straightforward things like primary repair of PV/SMV or patches. I think for anything more complex than that, either you have vascular help or you refer them out if you cant get vascular help. I'm sure there are some badasses who do it all (Doug Evans comes to mind) but thats the exception.