The big thing was the ventilators imo.
Back when this started people were being placed in to induced comas and put on ventilators which seriously damages the body.
I remember there’s was a dramatic ratio of people being placed on ventilators and dying.
Since we learnt that, people are now on less invasive breathing assisting apparatus and it is working.
Plus the steroids and drugs which have been found to help, it is little surprise death rates are declining.
We have recovered from many pandemics in the past(with less technology and medical knowledge/science) why should Covid 19 be any different?