Sciences are defined by the object of study, and by no means primarily by the method.
So, if neurophysiology studied processes in the nervous system, and neurobiology, for example, its structure and evolution (in fact, this also study, but not the fact that within the same discipline) - these would be different sciences. The methods of study in relation to the object are secondary: the earth's crust (for example, for clarity) can be studied by echolocation, drilling, electrical, chemical, seismic methods - but the object does not change from this. Therefore, there is only one science - geology.
However, even now, according to some definitions, neurobiology is a more general discipline than neurophysiology:
Neurobiology is a science that studies the structure, functioning, development , genetics, biochemistry, physiology and pathology of the nervous system. Neurobiology is also associated with neurophysiology, the latter studies the features of the course of physiological processes in the human brain and the resulting change in mental activity.
Note that this is only one of the possible definitions.
By and large, nothing. Nevertheless, historically, physiology has developed over many decades due to electrophysiological methods (EEG, microelectrode methods, potential research, etc.). Therefore, until now, the concept of "neurophysiology" is associated with the use of this group of techniques.
Perhaps this is why the term "Neurobiology", which is usually used in connection with biochemical or neuroimaging (MRI , PET, etc.) by research methods. However, I repeat, there is no fundamental difference between these concepts. In the vast majority of cases, they are interchangeable.