I have two feral cats adopted at different ages and time cuddled up with me right now.
One was found about 4 weeks of age. He is about 8 years old now. The other moved in at around 4 month's old, she is about 3 years old now. She only stopped acting feral and turned into a cuddly cat a few months ago.
When you finally manage to earn their trust is the greatest feeling.
Give them food and make sure they see you giving it to them. Sit in view of the food bowl. Move the food bowl closer to you daily. When the food bowl is right next to you, bother them by touching briefly. Do this until they no longer get scared of the touching and you can pet them for prolonged times.
This might be weird but you might want to separate kitten from mother to work solo with it as the mothers fear or hate to people can be learned quickly by the kitten. Older cats are also a lot harder to get tamed.
Source: I have two 8-12 week old kittens in a spare room and am working with them daily. I found them in my crawl space a week ago and took the mother to the vet to be fixed and let her go. They make progress slowly as they learned to be afraid of me from their mother.
My town does fostering for a couple reasons. Military people who get deployed, and also when the shelters are at capacity. You can foster animals until they are adopted permanently, and it helps the shelter because you are paying for their food and care. It can be heart breaking, though, but worth it.
Fostering is when an animal needs to be cared for outside of the shelter. Sometimes it is because the animal has a communicable disease. Sometimes it's because the shelter is too full.
So the foster "parents" take these animals, knowing (and hoping to an extent) to have them only for a bit. Until they get healthy and or adopted.
I adopted a cat from a humane society and my cat was at a foster home. The "parents" were a retired couple who loved animals. They cried after I adopted the cat, but said they knew it was for the best. I regularly sent them updates on how we were doing (with pictures of course).
The shelters give the foster "parents" supplies, food, medicine, etc to adequately care for the animals, and to encourage fostering.
A lot of fostering is by people with time and skills to handle animals with issues: kittens and puppies that aren't weened yet or older animals that need special medical care to get them back on track. Still others may need to learn to trust humans again due to coming from a bad situation. People usually won't adopt animals that are "broken" (need a lot of vet care at first or socialization work). And the average person doesn't have skills to handle circa eight week old baby animals.