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maybe an analogy will help you understand
it is not a guitar tuner that determines if the note is a or g
it is a guitar amplifier that will take 1 decibel molehill non threat to health
and make it 1000 decibel mountain .
Yes and? Not that different conceptually from when they would swab your throat 40 years ago, then put the swab in a petri dish, and wait a day to see what grew. Easier to detect strep after you give it some time to grow and reproduce - same with virus. They are small, there isn’t a lot of it in a sample, so you grow or reproduce it to make it easier to see.