Are the packs initially present inside the body or is it formed by exercise?If so how is it formed?
The Rectus Abdominus contains tendinous intersections that go across the muscle belly, dividing the rectus abdominus into 6 or 8 "packs". Whether you have a 6-pack or an 8-pack is determined by your genetics.
A combination of diet and exercise reveal the abs. Strengthening the rectus abdominus and surrounding musculature will improve the relative size of the packs. If you don't see the individual packs it is likely because you have too much subcutaneous fat and will need to burn that fat to see the 6-pack.
Are you talking about a "six pack"? That's essentially one muscle, called the Rectus Abdominus. They are always there. You can induce hypertrophy, an increase in size (of the muscle cells), but that's limited. As someone stated in another post, abs are built in the kitchen before they're built in the gym. As a strength professional, I lean away from sits ups and similar exercises because that's more hip flexion than it is core work.
Planks are your friend. You'll hit your core during EVERY exercise, and it's impossible not too. It's essential for stability.
Keep that in mind-if you decrease stability (closer stance, bosu ball), you'll increase
I assume you are referring to the six-pack? In which case they already exist as your abdominal muscles, its up to your diet and your exercise to accentuate them. You must also consider your diet + exercise because as the old saying goes "abs are made in the kitchen", meaning you must get to a very low body fat % in order to see the striations along the "packs"...means you have to get lean.
Also consider that different ab exercises will accentuate them in different ways. For example, weighted crunches are going to give you bulky looking protruding ab packs, where-as high volume high reps or endurance ab work will trace your ab's against the skin that covers them, instead of (looking like) pushing out of them, if that makes sense.