You can't save them all. Thats the fact of it. Even when cats are rescued, many of them die. Kittens that end up abandoned by the mother or dont have one are at the highest risk of euthanasia at a shelter because there is rarely enough resources to bottle feed babies every two hours and they get euthanized because that's less cruel than having them die out in the elements. Even those kittens, when fostered and fed and kept warm- they die. More often than you might think.
But you can save a cat or two. Or a lot of shelters have places you can drop off donations of food or other things. I had a cat with end stage kidney disease. After he died, I had a bunch of special food and meds leftover. donated them to a shelter and another cat who also had kidney problems, was able to stay in the shelter long enough get adopted because of that donation.
Then I adopted a couple of kittens from a TNR group. The mom was feral but the kittens got rescued. Mom is probably dead, as feral cats have astonishingly short lives, but my boys are happy and safe.
You can't save them all. But you can do your best and for the cats you can help thats all they need.