What is the purpose of multiple layers in the PCB? What do they need?
One way to think about tracks on a PCB is to compare them to highways in large busy cities. For example, in Dallas, Texas, I saw interstate highways five stories high, hundreds of feet high!
Traces of printed circuit boards are a bit like these super highways. Each lane usually only travels in one direction (well, many pcb tracks can travel in both directions, but otherwise, this is a good analogy). Parking lights can only slow traffic flow. If you want to travel from one part of a big city to another a few miles away, you may have to use a flyover or underpass to avoid traffic on other highways that you intersect with.
On the PCB, a function called a via connects the trace from one layer to another. Just like the traffic in small towns, the rails at the top and bottom of the board where the city is located tend to have many directions in the direction of the road - take curves and roundabouts as needed. But once they reach a channel they connect to another layer (in this analogy, the channel is considered an underground passage, an overpass, or just a manhole!), the inner lane usually acts as an interstate highway, only from the circuit One edge of the board to the opposite edge of the board. Unlike the top or bottom track where the city is located, the inner layer is usually assigned the direction. They are either fast-tracks from north to south or fast-tracks from east to west. On the military plates of missiles, they are usually round rather than rectangular, expanding the circle. On the interstate highway, these are highways with three digits starting with "2". The higher the first number, the farther away from the city center.
If it helps, here is the 3D image of the PCB I designed with 3D. The solid part of the board made of a fiberglass and epoxy (sometimes called "FR4") has been made transparent so that you can see the copper highways it contains. The gray vertical column is a via and electrically connects the green horizontal trace to the other layers through the via. In this particular screen shot, the purple and light green layers are not traffic layers, but are interrupted from this analogy, called the power plane. They are copper layers that help traffic lanes see headlights (reflections) from other floors.
To see where the top or bottom layer might look like the city (integrated circuit and other components), here is a screenshot from the top of this particular board: Blue indicates the copper layer on top of the board where the component is soldered. The warm brown you see in the place is the power plane below the city, which distributes electricity, natural gas, water and other basic services to the city.
It is not much different from any industrial area in any big city. It has tunnels, overpasses, underpasses, beautiful office buildings, huge complexes, and occasionally some ugly towns.
Printed circuit board layout is a hybrid of civil engineering and industrial art. A sad reality of this profession is that it is STEM, it is visual, it pays well, you don't have to be a genius to practice it, but few people are studying how to do it. Most of those living in poor countries with overpopulation will do everything they can to find a better life.
People who like art and can learn STEM courses for a long time can practice this art for the rest of their lives and pay a lot for their efforts. This special PCB is used for open spine surgery to help repair the spine of injured athletes, car wreckers and other accident victims, as well as the spine damaged by birth defects.
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