There was a nucleotide sequence in the covid strain that did not show up in any of the proposed hosts or progenitor viral sequences, which is where leaked documents showed NIH (Fauci included I believe) discussing the non-natural origin of the nucleotide sequence. It's possible to search for articles about the Fauci NIH emails, and whether they mean anything scandalous.
Here's a technical article at NIH discussing the theory of no known natural origin for a nucleotide subsequence