The test detect antibodies in the blood which indicates a recent infection in the few weeks before but it won’t detect the memory cells which is a type of long term immunity
But this test is very reliable for having immunity
Meaning if you have high numbers of antibodies circulating in the blood you are immune and if you have low numbers it doesn’t mean that you are not immune because the antibody levels goes down dramatically after few weeks and is stored in the memory cells if you want which will kick in any minute the virus hits again
Now watch them say it doesn’t mean immunity and you need vaccinations
Natural immunity is the best