In general, you can start doing gentle core and pelvic floor exercises a few days after delivery.
You should avoid traditional ab exercises like sit-ups. Instead, you can begin with some basic core stabilization exercises.
The posterior pelvic tilt is a very simple exercise that activates your deep core muscles and realigns your pelvis
Diaphragmatic breathing is another simple exercise you can do immediately after your delivery to reactivate the abdominal muscles. Many women do it wrong.
The best exercise to strengthen these muscles is the Kegel exercise. But you must do it properly.