PCR is an amplification system that can multiply the molecule you look for. Every cycle you run it, you double the sample. At a certain point, you can run it till you have detected a single molecule in the sample. This is what the tests were doing, and why having a positive test did not mean you actually were infected, rather that it found a single suspicious molecule in your sample. It was qualitative, not quantitative.

Covid PCR tests looked for pieces of Coronavirus. However, common flu also contains many types of Coronavirus.

So, the Covid PCR tests would give a positive result if it found a traces of dead flu virus. That's why the Covid numbers were so high in 2020/21, and there were no flu infections in the same time. They wrote down all Flu cases to Covid.