Let me talk about sit ups first.
The purpose of sit-ups is to sit up, so the whole torso should be up when doing this movement, and you cannot pull yourself up with the abdomen alone, and you must pull yourself up with the strength of the legs and waist.
The curled belly just bends the abdomen, mainly using abdominal muscles.
If you want to practice abdominal muscles, sit-ups are basically ineffective.