In Material properties in editor there is Shader property. For 3D models You can choose there new spatial material or new shader material. Spatial material is built-in shader with code hidden, it is that thing You can choose albedo color, roughness, metallic and many other properties. When You choose to use it and create new spatial material, You can set all its properties as You like, and when You click Shader property again, You will notice a new option there - convert to shader material. This will not change any properties, and You will have access to spatial materials code, all properties will be transormed into its uniforms. The same applies to particles material.