On my 2019 there is what's called an "after blow" setting to reduce humidity buildup that has to be activated by the dealer. Ask them about this. What it does is run the fan after the car is off for a few on/off cycles. It's not a cure, it's a preventative. If there is already mold you'll have to get it cleaned out.

If it happens only while the car is in auto stop (most cars do this) then turn off the auto stop.

 

Not sure what area of the country you purchased it in, but back some weeks ago in August, there were unpressidented flooding in some large midwest cities. I would check to see where the vehicle came from.
If that info is accurate.
It could be just something in the HVAC system. . . but check under floor carpeting, evidence of of water stains, etc.

 

I am certainly no expert, but I had had a similar issue with a 2014 Chevy Malibu and now my 2020 Terrain SLT. In my case I think it is related to mold and mildew that apparently builds up on the evaporator core (if that is the correct terminology). On the Malibu, I was told to turn off the A/C a few minutes before getting to my destination to let things dry out a little bit before parking. I don't believe that either of my cars had/have the "after blow" feature. I have fixed mine temporarily by spraying almost a full can of Lysol (or similar) into the input vents under the hood (where the wipers park), and also replacing and spraying the cabin air filter. My wife says it smelled like "dirty socks", which was pretty accurate.

 

I'm 100% positive the 2020 has the after blow feature. Again, it's a dealer setting. You can not turn it on or see that it exists yourself.

 

Five years ago, I bought a 2016 Equinox with 107,000 miles on it, that had a terrible odor. Smelled like feces. The seats were somewhat stained, so I thought they were stinky. Turns out, that wasn't it. One day, I had the idea to spray Lysol into the heater blower intake, and that fixed it. No more foul odor, and it's been good ever since.