Actually, they test for genetic sequences, sequences that don't actually have to come from any alleged biological virus. Honestly, before Covid, I believed in viruses myself. It was only after it started researching it that a journalist friend of mine first introduced me to the fact that some were questioning the existence of not just the Cov 2 virus, but all viruses. At first, I was skeptical and I told her so. Ironically, she got back on the bandwagon that viruses are real, but I was eventually persuaded that they weren't.
As to the notion that HIV causes AIDS, there's plenty of evidence that this isn't the case. After a bit of searching on the internet, I found the following article that gets into a fair amount of this evidence:
Questioning the AIDS Virus, HIV, and AZT Controversy | shirleys-wellness-cafe.com
As to the evidence that there are no pathogenic viruses, at present my favourite essay on the subject is the following one from Dr. Mark Bailey:
A farewell to virology (Expert Edition) | drsambailey.com
Below are some articles the focus almost exclusively on the alleged Cov 2 virus.
This one was done by a journalist who started looking into it:
COVID19 – Evidence Of Global Fraud | Off Guardian
Dr Stefan Lanka's article on the alleged Cov 2 virus is also very interesting in my view:
Misinterpretation VIRUS II (2) by Dr. Stefan Lanka – Beginning and End of the Corona Crisis | yummy.doctor