How/Why does M_Tech_Hex_Tile material work?

After studying a simple chapter (24hr book) on materials, I opened M_Tech_Tex_Tile and tried to read it. Google gives me a little bit of information on specific blocks, but nothing about why they are connected together, or how they work.

“Metallic from mask channel” is a Lerp with only one of three inputs connected. How can it interpolate from A to B if there is no A or B input? What is it doing?

Why does “Mask texture” need the BumpOffset input?

“Mask out detail normal between tiles” does a Lerp between two textures that look identical. What is that for?

I have “Game Development and Simulation” which has two big material chapters. Where else can I learn to anticipate what the combinations of material operators can do?