Nephrologists are a type of physician. A physician is a hospital-based doctor that does not do operations (like a surgeon).
There are many different types of physician.
But nephrologists are experts in the medical (i.e. non surgical) treatment of kidney diseases, including:
Most hospitals do not have a team of nephrologists partly because the diseases they deal with are rare. They tend to work in the larger teaching hospitals, but often do clinics in local hospitals where you can see them.
There is a lot of overlap between CKD and heart disease and diabetes.
So nephrologists work closely with other hospital-based physicians including heart and diabetic specialists (called cardiologists and diabetologists).
They also work closely with different types of hospital-based surgeon. These include urologists (surgeons that do operations on the kidneys and urinary tract), and vascular (blood vessel) and kidney transplant surgeons.