The PCB as a whole? Usually fiberglass or some other substrate to which sheets of copper are fastened. Then wherever you don’t want a copper trace is removed by acid. Then the board is covered by something that acts as a protective layer to prevent oxidation.
Today it can be just dye, but solder masks used to be naturally green due to the nature of how these boards were made.
But what is it, aluminum?
The PCB as a whole? Usually fiberglass or some other substrate to which sheets of copper are fastened. Then wherever you don’t want a copper trace is removed by acid. Then the board is covered by something that acts as a protective layer to prevent oxidation.
Here’s a video detailing the process for those interested 😀
https://youtu.be/YJr-kHy6STg?si=CugU2ezd6KZI5wDA