What activities do national defense students have to practice in the army?

Nothing, depending on what kind of treatment the army gives you, whether it's a soldier or a platoon leader, you won't be asked to do much work. Just study more and see more. It's similar to a one-day tour of a local school barracks, except that you are on an N-day tour.

It is different to be assigned to a company after graduation. You really need to be an officer. I suggest you ask more questions with an open mind and don't be afraid of jokes. After all, you really don't understand.