Need guidance on taking care of abandoned kitty

Age:17 Country: brazil I live in a farm, and there's a lot of strays around here. Me and my sister found a litter of newborns, but since they looked ok we decided to just wait for their mom to come. After a while, we went back to that spot but only 1 kitty was in there and not the other ones, I'll call them star. We waited around more for their mom, but after showing them to (literally) all of the female cats in the farm, I concluded that star must be a rejected cat. I couldn't stand to just leave them there so I took them into my room and me and my sister started taking care of it.

We unfortunately don't have any other way of feeding the kitty since my family is dead broke, so we used powdered milk mixed with ambient temperature water, yes I know it's probably not good but it's the best we have. that's also why we can't get star to a vet. Apparently there's also no rescue thing in my city too, so the best option is just to take care of them myself.

Star is probably a few weeks old, their eyes are already opened and they can walk around decently well. Now for the actual questions: I was giving them ~3 ml of milk every few hours. Should I give them more? Is there any other thing she can eat that's easy to make/find that's not powdered milk? Does the composition of the milk matter a lot? We have been feeding them for a day now but I still haven't gotten them to poop. I've tried stimulating their butthole with a qutip and a few wet cloths but nothing worked and I'm also getting very concerned about that. Is this normal? Is there a better way of stimulating them? Me and my sister let them walk around after we feed them, and since my room is small and boring there really is no risk of anything bad happening to them. Should we do something else with them, like try to play? or any other important activity in this stage of development?

Other info: I don't know their sex, they're a black cat with a few white/yellow spots(I'm sure is just part of their fur), they sleep in a small blue cardboard box with a bunch of soft blankets. They spasm sometimes when sleeping, idk if that's bad. We feed them with a very small syringe.

I also apologize if I seem like a bot/dumbass, I'm very autistic and I really would do more for star if I didn't depend on my family.