A baby should be kept in the car seat in the stroller until they can hold their head up on their own and sit up without assistance.
Usually the baby can be moved to the stroller without the car seat by age 5 to 6 months of age.
Usually by the time the baby is 5 months old they can be put into the stroller without the car seat as they're usually able to sit up on their own by then.
Just use your judgement as babies are different but as long as you keep the car seat in the stroller until the baby is 5 months old then it should be okay to remove the car seat from the stroller and let the baby sit in the stroller without the car seat.