Apply shader to whole object

I’m completely new to shaders and I don’t think that I really understood all of the concepts. To use a shader, I have to use a material file. But what if a models has several different materials and I want to use the shader for the wohle model? If I wrote a dissolve shader for example, how could I apply it to the whole model, I imported from Blender, and how would I keep the lighting and the textures of the model as they are?

Sorry, this is probably really easy, but as I said, I don’t really understand the whole concept.