Simple stuff...
Old electronic products, like TVs had tubes and the sockets were riveted onto a metal chassis. All the parts & wires were connected "point-to-point" between the terminals of the sockets. This was very labor intensive, slow and error prone.
Then, the evolved into printed circuit boards. The simplest boards have one side plain and the connections on the other side. Modern circuit boards are made of epoxy resin and fiberglass, similar to the skin of a power boat or corvette, except flat.
The board comes with a solid sheet of copper on the board. They put a photo-sensitive chemical on the copper, put a clear mask having the desired wire connections over the board and expose the chemical to a bright light. They develop the board, leaving only chemical on the areas where the wires are to remain. They dip the board in acid to remove all the unwanted copper.
Most circuit boards have circuits on both sides. After they drill the component holes in the board, they plate it.. this puts a very solderable coating on the copper. It also covers the insides of the holes, making connections from one side to the other. A hole which simply passes a signal from one side to another is called a "via" or "feed thru".
The more complex your electronic circuit, the more wires you will need. At some point, you had to go from 1 side to 2. When things get more complex, they sandwich multiple thinner boards together which allows inside layers. Some circuit boards may have 20 or more layers. The holes in the board make the connections through the layers. Sometimes, the inner layers may be nearly solid for power or ground. Sometimes they look like the outside layers, and you can hold the board up to the light and see the inner layers.
Once a bare board is manufactured, it is tested in a machine to make sure only the desired conections are there, there are no shorts and no opens. Once it passes, it goes to the customer.