It's more like covid evolved in its host species to a form where it was able to affect humans.

To oversimplify things:

Picture a virus as a really funky key. Your cells (animal cells) are covered in funky looking locks. These locks are meant to allow good things in while generally keeping bad stuff out, all throughout your body. When a virus mutates, it is finding a way to spread more efficiently and effectively. One of these methods is by changing binding protein shape to fit more locks.

Similar situation with swine flu, avian flu, and many other zoetic viruses. We keep an eye on which animal viruses are getting dangerously close to making the human jump by taking animal cell lines, replacing the receptors (locks from earlier) with human versions, and then seeing if the virus is able to bind and replicate.