It was either in Bats or Pangolins. Both are known to be hosts. The jump to humans wasn’t hard in a place where they trade in live animals as food without supervision.
Cross-species infection has been know to happen numerous times. Bird Flu, Swine Flu, SARS. etc.
This one jumped and was spread faster than it was detected and turned out to be more communicable and deadly in humans than it is in animals.
It looks like the animals that carry it are largely unaffected by it making it hard to see that they had a sick animal on their hands in the food market.