Hello, and welcome!
Bless you for rescuing sweet Rosie from the streets and giving her a loving, forever home!
If her litterbox always smells of urine, despite your constant scooping, she may be urinating more frequently and if she is 4 months old or over, she very possibly could be in heat. Cats in heat typically urinate more frequently. Symptoms of heat generally include: being very affectionate, very vocal, rolling on the floor, laying on the floor with their rear end up in the air, walking with their tail straight up and a little to one side.
Even though you're scooping daily, the urine is soaking into the plastic of the litterbox and the box needs to be washed out with mild soap and warm water at least twice a week. Try to scoop more than once daily. Sometimes, the urine smell will never come out of the plastic material of the box. In this case, you can look for a better litterbox made of a type of material that is more efficient in repelling the odor and not allowing it to soak in, with frequent washing. You might also want to try litterbox plastic liners, but if Rosie has front claws, they might tear the plastic when she scratches. Make sure you give her quite a few inches of litter to scratch in.
Another reason the box may smell so strongly, is that she has a urinary tract infection and her urine will smell stronger, if this is the case. If she's not exhibiting any symptoms of discomfort upon urinating, running to the box every few minutes, straining to go, blood in the urine, that's great! Then, I would attribute her urinating more frequently and/or a stronger smell to her urine if she is in heat and/or if she is not drinking enough, is slightly dehydrated and her urine is more concentrated.
Since you brought her in from the street, it would be advisable to have her evaluated in person by the vet, who will run a urinalysis and also let you know if she is in heat.
I'm assuming Rosie is an indoor-only cat now, so she doesn't risk pregnancy, if she is in heat and if she is not in contact with any unneutered males, but it is healthier for her to be spayed, when the vet feels it's the right time, around 5.5 months of age (between heat cycles, if she weighs enough and has no health issues), so discuss this with the vet.
I hope all will be well with Rosie!
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