A lot of the plant is similar to other power plant types. Normal engineering methods in Civil, Mechanical, and Electrical engineering apply. The main difference is the reactor itself, which requires a very large strong pressure vessel made from steel, which is assembled in sections welded together, and the primary circuit which circulates water through the reactor and the steam raising boilers. There are three or four primary circuit loops, these are made of large diameter steel tubes in sections welded together. One of the most important jobs in building the reactor and primary circuit is to ensure that all the welds are perfectly formed and a lot of inspection techniques are used to ensure this. A very important part of the vessel is the head, which is detachable for fuelling and carries the control rod mechanisms (in a PWR). You can read more about the reactor design in the link below.