Whether you hold your head or put it on your ears, it is important to sit up with your abdomen instead of holding your head up with your hands. This will not only lock the target muscle, but also damage the cervical spine, so you must not sit up with your hands, but rely on the contraction force of your abdomen.
3. If you use sit-ups to practice your lateral abdominal muscles, you can turn your body sideways during sit-ups, but there are many better ways. You can buy some books for reference.
4. Push-ups mainly exercise triceps brachii, which has little effect on chest. If you want to exercise the lower chest muscles, you can exercise the whole chest by pressing on your back, with your head down and your feet down.