Like this one [here](https://www.auspienterprises.com/wp-content/uploads/2021/12/PCB1-1.jpg)
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Some are holes that are meant to have components leads through them. Others are meant to just connect the point you see to another layer of the board. This lets printed wires cross other wires easily.
Older simpler boards have just the two sides. Newer higher density boards can have several internal layers, all with their own tracks, that you don’t see.
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