In status quo, doctors can undergo specialization training or opt to be generalist physicians.
In the former, doctors specialize in major fields of medicine, such as Internal Medicine (IM) and General Surgery (GS), and they may further sub-specialize within those fields, such as Cardiothoracic Surgery after training in GS. It is uncommon for specialists to switch from one specialty to another (i.e., becoming a surgeon after training in IM).
In the latter, doctors have a broader understanding of basic diseases, albeit lesser in depth, due to limitations in resources and time. Instead of being surgeons or anesthetists (and other specialists), general physicians act as primary care doctors or family doctors.