It’s the doctor whose entire specialty is the general environment of the inpatient hospital, they also might be called an “internist” as in “internal medicine.” The outpatient equivalent is a primary care, general practice, or family physician, since they both handle all body systems at once. The hospitalist is usually the attending / resident for an admitted patient for something basic like pneumonia or early stage heart or kidney disease (or some complicated combination of multiple that’s not well handled by any one specialist). Once those are advanced in one particular organ or system they get taken by a cardiologist or nephrologist etc but for most patients the hospitalist is the doctor in charge. In psychiatry the psychiatrist is the attending / primary team, but the hospitalist is consulted to do a brief medical assessment to handle any underlying medical conditions like diabetes so that the psychiatrist doesn’t fuck up their insulin or heart medicine.