What do you think they're testing?

They are checking if they can find virus DNA fragments into sample from your internal fluids.
They are using RQ PCR technics to do that.
Any DNA fragments from anywhere can contaminate the sample and get false positive
If the sample is not representative of your body, you can get virus and be a false negative.
Precision of this test is very weak.
They are not testing your ability to naturally fight against CoVID-19.