Alright yall I'm about to hit you mofos with some fact bombs.

DNA polymerase is an enzyme that takes in a strand of DNA, unzips it, and turns it into two copies of itself. (More accurately, the unzipping is performed by a different enzyme, but they work together). "But NotFooled," you might ask, "how does the DNA polymerase avoid just copying every single piece of DNA it ever comes into contact with?" Great question! DNA polymerase cannot start the process of replication unless another enzyme called a primer binds with the DNA first. There are a lot of different types of primers, and based on the configuration of proteins within them, they can only bind to specific DNA sequences. This is how your body (and those of a lot of other lifeforms) knows when and which DNA to replicate at the correct time.

PCR works by using this process to replicate specific target DNA strands a bunch of times exponentially. First, genetic material from the test subject (for Covid, usually from a nasal swab) is put into a test tube with DNA polymerase and a set of primers keyed to the Covid genome. This way, ONLY Covid DNA will have primers bound to it, so it's impossible for the RNA polymerase to copy any other DNA. Then, copying occurs for a bunch of cycles. At the end of these cycles, if there's a shitload of Covid DNA in the test tube, the only way it could possibly be there is if there was Covid DNA in your system already. If there weren't any, the primers wouldn't have bound to anything and there wouldn't be any copies.

To rectify the book analogy, here's an analogy to explain PCR: You go into a bookshelf in a stranger's home, testing them for communism. You have a team of workers who are all searching for The Communist Manifesto. If they find that specific piece of literature, they make a copy and throw it onto the floor. At the end of the day, there will either be a lot of Communist Manifestos on the floor, or there will be nothing.

The one nugget of truth I've seen in this comment section is that PCR can be used to detect nearly anything in the body. This is true, simply because you can perform it with any primers. You could use primers keyed to the flu, or covid, or ebola, or human DNA, to try and look for any of those things. But you'll only find them if they're actually there. You're not going to be able to get a positive result for anything you want. If this technology were that shitty, it wouldn't have won a Nobel Prize and it wouldn't be the gold standard for testing throughout the world.

Sources:

Bioengineering 80 at Stanford

Professor Drew Endy's office hours

Wikipedia

Common Sense