look at any channel, RGB, metal, specular as a value from 0-1 and from whatever maps you sample from incorporate math that would otherwise modify it as you see fit. for example, if you had a wetness function you might take the specular and add a value to it, like a rainyness (scalar parameter) value. this would make it shiney, for example.
What exactly you want the material to do functionally is going to determine what maths you include. This is where the artistry comes into play. I’m not going to be able to tell you what to do specifically, but I can suggest you try this guys series, it’s what I learned materials from: Landscape Material Tutorial Part 1 (Unreal Engine 4) - YouTube
As far as the master-material, you make a decision on what you want to have a set of features/qualities about your game. Do you include roughness maps, do you use a flat value, do you tweak your normal-maps or add detail blending. Some basic ship-of-the-line set of my game will look thusly, and make a core set of functions around that. For me, I made a global snow function that put snow on meshes, but also can be applied to landscapes and other things so my snow is all (visually) consistent across all my stuff.
What I am describing is a modular approach and that is valuable into and of itself, but it’s also how I did it for my project b/c that’s what made the most sense for me. How you want to break things down will largely depend on what you do. If you are making a space-combat game, maybe you don’t need any insight into landscapes, etc, etc. Game functionality will have a large weight here as materials can do quite a bit and in some cases even mirror/replace some kinds of logic, like using WorldPositionOffset to rotate a mesh vs needing to update the instance transform: https://www.tomlooman.com/unreal-engine-material-vertex-shaders/