The paper I attached says pigs don't get it.The source is bats. We are getting it from bats , No intermediary host has yet been found. Many suggestions have been made but we are getting it from bats. The prime target was wet markets with its bat guana. The other source could be bat guana fertilizer. So far, it looks like a direct infection. The main target are the cells with ACE2 and that is gram negative bacteria. They have the highest source of ACE2.
When the numbers of anaerobic Gram negative rod-shaped bacteria and spirochetes increase compared to the other bacteria in the sub-gingival plaque then gingivitis is more likely to occur. The organisms in this plaque contains large numbers of gram negative proteolytic and hemolytic bacteriathat cause inflammation of the gingiva.
Hence, wearing a mask. Could the source be bat guana. Yes and it could be airborne. Anything that has alot of gram negative bacteria is a target because of the ACE2. The question is where is the greater source of infection . That is what the scientists cannot figure out. Pigs can get it - depends on diet and there is a lot feces in a pig pen- lots of gram negative bacteria in stool. That is what they find in people - covid 19 is in the stool. Look at a nursing home and just how much of a problem that is.