Most of the flights are cargo or passenger planes with no passengers but full cargo holds - price of air freight has soared(sic.)which makes it economical to do this - also means pilots and planes retain their license to fly - pilots must do at least 3 take offs and landings every 90 days to retain their license. Airlines need to have a cohort of licensed pilots to fly when the restrictions are over.
Are you sure when the flight numbers are passenger flights and aren't specific freight planes ?
Here in Colorado Denver airport is a United hub and we have a number of friends who are pilots on both domestic and international passenger flights. Domestically they are flying mostly empty planes from the passenger perspective but full of cargo, internationally they are the same but need to keep the flights otherwise they could lose their slots at foreign airports unless the rules change. However they also say that the number of flights have been dramatically reduced, I was shown a picture of Denver Airport with what looked like around 50 aircraft parked on the apron with their engines covered.
