You can divide a processor into two blocks connected to each other: the control and the data path. A 1970s book illustrated this as a puppet controlled by strin

You can divide a processor into two blocks connected to each other: the control and the data path. A 1970s book illustrated this as a puppet controlled by strings, with the puppet being the data path and the hands at the other end of the strings as the control. How many strings do you need? The data path has a bunch of registers, things like an ALU, busses and multiplexers. You need a "write enable" wire for each register (or a single enable plus a small number of address bits), select signals for the multiplexers and some control bits for the ALU. 21 bits is as reasonable a number as any other. A slightly different data path might require fewer or more control signals even when used to execute the exact same instructions. The metal layer is deposited on the whole wafer, but then everything is etched away except where you want wires. So the "wires" and the "metal layer" are indeed the same thing. The vias are just little wells in the oxide beneath the metal layer. So when you deposited metal on the whole wafer some of that filled in the little well making a vertical connection between the wire and what was under the oxide.